diff --git a/app/(dashboard)/settings/actions.ts b/app/(dashboard)/settings/actions.ts
index 7d21ede..c585e18 100644
--- a/app/(dashboard)/settings/actions.ts
+++ b/app/(dashboard)/settings/actions.ts
@@ -12,13 +12,21 @@ import { getServerConfig } from "@/server/config";
 import { getBrandingService } from "@/server/branding/runtime";
 import { getSqliteConnection } from "@/server/db/client";
 import { appProfiles } from "@/server/db/schema";
+import { runtimeEvents } from "@/server/db/schema/runtime";
 import { domainActorFromSession } from "@/server/auth/domain-actor";
 import { getFileService } from "@/server/files/runtime";
 import { getPublicAiSettings, updateAiSettings } from "@/server/settings/ai";
 import {
   getUserPreferences,
+  updateLanguagePreference,
   updateColorModePreference,
 } from "@/server/settings/preferences";
+import { buildLocaleCookie } from "@/server/i18n/locale";
+import {
+  ContentTranslationService,
+  parseContentTranslationsFromFormData,
+} from "@/server/i18n/content";
+import { getReferenceTranslationKeys, I18nService } from "@/server/i18n/service";
 import { requireFreelancerBackend } from "@/server/web/freelancer";
 import { cleanText } from "@/server/web/form-data";
 
@@ -28,6 +36,13 @@ export async function loadSettings() {
   const ai = getPublicAiSettings(actor);
   const preferences = getUserPreferences(actor);
   const branding = getBrandingService().getPublic();
+  const i18nService = new I18nService(getSqliteConnection().db);
+  const locales = i18nService.listLocales(actor);
+  const i18nSettings = i18nService.getSettings(actor);
+  const translations = i18nService.listUiTranslations(actor);
+  const completion = i18nService.getCompletion(actor);
+  const contentI18n = new ContentTranslationService(getSqliteConnection().db);
+  const brandingTranslations = contentI18n.listEntityTranslations("branding", "default");
 
   return {
     firstName,
@@ -46,6 +61,18 @@ export async function loadSettings() {
     hasCustomLightLogo: Boolean(branding.lightLogoFileId),
     hasCustomDarkLogo: Boolean(branding.darkLogoFileId),
     hasCustomFavicon: Boolean(branding.iconFileId),
+    language: preferences.language,
+    i18n: {
+      locales,
+      defaultLocale: i18nSettings.defaultLocale,
+      catalogVersion: i18nSettings.catalogVersion,
+      translations,
+      completion,
+      referenceKeys: getReferenceTranslationKeys("all"),
+    },
+    contentTranslations: {
+      branding: brandingTranslations,
+    },
   };
 }
 
@@ -143,6 +170,140 @@ export async function saveColorMode(colorMode: string) {
   }
 }
 
+export async function saveLanguagePreference(language: string) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const preferences = updateLanguagePreference(actor, { language });
+
+    (await cookies()).set(buildLocaleCookie(preferences.language));
+    recordI18nEvent(actor.authUserId, `user_language_updated:${preferences.language}`);
+    revalidatePath("/", "layout");
+    revalidatePath("/settings");
+    return { success: true, language: preferences.language };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Dil tercihi kaydedilemedi." };
+  }
+}
+
+export async function createLocaleAction(input: {
+  code: string;
+  name: string;
+  nativeName: string;
+  fallbackLocale: string;
+  textDirection: "ltr" | "rtl";
+}) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    const locale = service.createLocale(actor, input);
+    recordI18nEvent(actor.authUserId, `locale_created:${locale.code}`);
+    revalidateI18nPaths();
+    return { success: true, locale };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Dil eklenemedi." };
+  }
+}
+
+export async function updateLocaleStatusAction(code: string, status: "draft" | "active" | "archived" | "test") {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    const locale = service.updateLocale(actor, code, { status });
+    recordI18nEvent(actor.authUserId, `locale_status_updated:${locale.code}:${locale.status}`);
+    revalidateI18nPaths();
+    return { success: true, locale };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Dil durumu güncellenemedi." };
+  }
+}
+
+export async function setDefaultLocaleAction(code: string) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    const settings = service.setDefaultLocale(actor, code);
+    recordI18nEvent(actor.authUserId, `default_locale_updated:${settings.defaultLocale}`);
+    revalidateI18nPaths();
+    return { success: true, settings };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Varsayılan dil güncellenemedi." };
+  }
+}
+
+export async function saveUiTranslationAction(input: {
+  locale: string;
+  namespace: string;
+  key: string;
+  value: string;
+}) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    service.upsertUiTranslation(actor, input);
+    recordI18nEvent(actor.authUserId, `ui_translation_saved:${input.locale}:${input.namespace}.${input.key}`);
+    revalidateI18nPaths();
+    return { success: true };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Çeviri kaydedilemedi." };
+  }
+}
+
+export async function resetUiTranslationAction(input: {
+  locale: string;
+  namespace: string;
+  key: string;
+}) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    service.resetUiTranslation(actor, input);
+    recordI18nEvent(actor.authUserId, `ui_translation_reset:${input.locale}:${input.namespace}.${input.key}`);
+    revalidateI18nPaths();
+    return { success: true };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Çeviri sıfırlanamadı." };
+  }
+}
+
+export async function exportI18nAction() {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    recordI18nEvent(actor.authUserId, "i18n_exported");
+    return { success: true, package: service.exportPackage(actor) };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Çeviri paketi dışa aktarılamadı." };
+  }
+}
+
+export async function previewI18nImportAction(rawJson: string) {
+  try {
+    await requireFreelancerBackend();
+    const parsed = JSON.parse(rawJson);
+    const localeCount = Array.isArray(parsed.locales) ? parsed.locales.length : 0;
+    const translationCount = Array.isArray(parsed.translations) ? parsed.translations.length : 0;
+    if (parsed.format !== "neta-i18n" || parsed.version !== 1) {
+      return { error: "Import paketi desteklenmiyor." };
+    }
+    return { success: true, preview: { localeCount, translationCount, defaultLocale: parsed.defaultLocale ?? "tr" } };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Import paketi okunamadı." };
+  }
+}
+
+export async function commitI18nImportAction(rawJson: string) {
+  try {
+    const { actor } = await requireFreelancerBackend();
+    const service = new I18nService(getSqliteConnection().db);
+    const result = service.importPackage(actor, JSON.parse(rawJson));
+    recordI18nEvent(actor.authUserId, `i18n_imported:${result.translations.length}`);
+    revalidateI18nPaths();
+    return { success: true, package: result };
+  } catch (error) {
+    return { error: error instanceof Error ? error.message : "Çeviri paketi içe aktarılamadı." };
+  }
+}
+
 export async function saveGeneralSettings(formData: FormData) {
   const uploadedFileIds: string[] = [];
   let brandingCommitted = false;
@@ -170,6 +331,9 @@ export async function saveGeneralSettings(formData: FormData) {
     }
 
     const brandingService = getBrandingService();
+    const contentI18n = new ContentTranslationService(getSqliteConnection().db);
+    const localization = contentI18n.getLocalizationContext(actor);
+    const brandingTranslations = parseContentTranslationsFromFormData(formData, "branding", localization);
     const current = brandingService.getPublic();
     const lightLogoFileId = await uploadBrandingFile(formData, "lightLogo", "branding_logo", actor);
     if (lightLogoFileId) uploadedFileIds.push(lightLogoFileId);
@@ -188,6 +352,7 @@ export async function saveGeneralSettings(formData: FormData) {
       ...(iconFileId ? { iconFileId } : {}),
     });
     brandingCommitted = true;
+    contentI18n.upsertEntityTranslations("branding", "default", brandingTranslations);
 
     deleteSupersededBrandingFiles(actor, current, updated);
 
@@ -306,3 +471,18 @@ function revalidateBrandingPaths(): void {
   revalidatePath("/portal", "layout");
   revalidatePath("/manifest.webmanifest");
 }
+
+function revalidateI18nPaths(): void {
+  revalidatePath("/", "layout");
+  revalidatePath("/settings");
+  revalidatePath("/portal", "layout");
+  revalidatePath("/api/v1/meta");
+}
+
+function recordI18nEvent(authUserId: string, message: string): void {
+  getSqliteConnection().db.insert(runtimeEvents).values({
+    type: "i18n.settings",
+    message: `${authUserId}:${message}`,
+    createdAt: new Date(),
+  }).run();
+}
diff --git a/app/(dashboard)/settings/page.tsx b/app/(dashboard)/settings/page.tsx
index 1f156fc..bebd4ce 100644
--- a/app/(dashboard)/settings/page.tsx
+++ b/app/(dashboard)/settings/page.tsx
@@ -5,8 +5,11 @@ import Image from "next/image";
 import {
   Blocks,
   Brain,
+  Download,
+  Globe2,
   ImageIcon,
   Key,
+  Languages,
   Monitor,
   Moon,
   Palette,
@@ -19,10 +22,19 @@ import {
 } from "lucide-react";
 import {
   loadSettings,
+  commitI18nImportAction,
+  createLocaleAction,
+  exportI18nAction,
+  previewI18nImportAction,
   removeBrandingAsset,
+  resetUiTranslationAction,
   saveAiSettings,
   saveColorMode,
   saveGeneralSettings,
+  saveLanguagePreference,
+  saveUiTranslationAction,
+  setDefaultLocaleAction,
+  updateLocaleStatusAction,
   updatePassword,
   updateProfile,
 } from "./actions";
@@ -34,13 +46,44 @@ import {
   Label,
   RadioGroup,
   RadioGroupItem,
+  Textarea,
 } from "poyraz-ui/atoms";
 import { toast } from "poyraz-ui/molecules";
 import { applyColorMode } from "@/components/theme/color-mode-sync";
+import { LocalizedFields, type LocalizedFieldValues } from "@/components/i18n/localized-fields";
 import { isColorMode, type ColorMode } from "@/lib/color-mode";
+import { contentTranslationRegistry } from "@/lib/i18n/content";
 
 type AiProvider = "groq" | "ollama" | "openai" | "gemini";
 type BrandingAsset = "lightLogo" | "darkLogo" | "favicon";
+type LocaleStatus = "draft" | "active" | "archived" | "test";
+type LocaleRecord = {
+  code: string;
+  name: string;
+  nativeName: string;
+  status: LocaleStatus;
+  fallbackLocale: string | null;
+  textDirection: "ltr" | "rtl";
+  builtIn: boolean;
+  sortOrder: number;
+};
+type TranslationRow = { locale: string; namespace: string; key: string; value: string };
+type ContentTranslationRow = { entityType: string; entityId: string; field: string; locale: string; value: string };
+type ReferenceKey = {
+  key: string;
+  namespace: string;
+  translationKey: string;
+  tr: string;
+  en: string;
+  parityOk: boolean;
+};
+type TranslationCompletion = {
+  locale: string;
+  translated: number;
+  total: number;
+  percent: number;
+  missingKeys: string[];
+};
 
 const colorModeOptions = [
   {
@@ -105,10 +148,32 @@ export default function SettingsPage() {
     favicon: false,
   });
   const [isSavingBranding, setIsSavingBranding] = useState(false);
+  const [language, setLanguage] = useState("tr");
+  const [locales, setLocales] = useState<LocaleRecord[]>([]);
+  const [defaultLocale, setDefaultLocale] = useState("tr");
+  const [catalogVersion, setCatalogVersion] = useState(1);
+  const [translations, setTranslations] = useState<TranslationRow[]>([]);
+  const [brandingContentTranslations, setBrandingContentTranslations] = useState<LocalizedFieldValues>({});
+  const [referenceKeys, setReferenceKeys] = useState<ReferenceKey[]>([]);
+  const [completion, setCompletion] = useState<TranslationCompletion[]>([]);
+  const [newLocale, setNewLocale] = useState({
+    code: "fr",
+    name: "French",
+    nativeName: "Français",
+    fallbackLocale: "en",
+    textDirection: "ltr" as "ltr" | "rtl",
+  });
+  const [selectedLocale, setSelectedLocale] = useState("fr");
+  const [selectedNamespace, setSelectedNamespace] = useState("navigation");
+  const [translationSearch, setTranslationSearch] = useState("");
+  const [editingValues, setEditingValues] = useState<Record<string, string>>({});
+  const [importJson, setImportJson] = useState("");
+  const [importPreview, setImportPreview] = useState("");
   const assetObjectUrlRefs = useRef<Partial<Record<BrandingAsset, string>>>({});
 
   const tabs = [
     { name: "Genel", icon: Palette },
+    { name: "Diller ve çeviriler", icon: Languages },
     { name: "Profile & Account", icon: User },
     { name: "AI Preferences", icon: Brain },
     { name: "Security", icon: Shield },
@@ -140,6 +205,15 @@ export default function SettingsPage() {
         darkLogo: settings.hasCustomDarkLogo,
         favicon: settings.hasCustomFavicon,
       });
+      setLanguage(settings.language);
+      setLocales(settings.i18n.locales);
+      setDefaultLocale(settings.i18n.defaultLocale);
+      setCatalogVersion(settings.i18n.catalogVersion);
+      setTranslations(settings.i18n.translations);
+      setReferenceKeys(settings.i18n.referenceKeys);
+      setCompletion(settings.i18n.completion);
+      setBrandingContentTranslations(toLocalizedValues(settings.contentTranslations.branding));
+      setSelectedLocale(settings.i18n.locales.find((locale) => !locale.builtIn)?.code ?? "en");
     };
 
     void fetchData();
@@ -254,6 +328,129 @@ export default function SettingsPage() {
     }
   };
 
+  const refreshI18nSettings = async () => {
+    const settings = await loadSettings();
+    setLanguage(settings.language);
+    setLocales(settings.i18n.locales);
+    setDefaultLocale(settings.i18n.defaultLocale);
+    setCatalogVersion(settings.i18n.catalogVersion);
+    setTranslations(settings.i18n.translations);
+    setReferenceKeys(settings.i18n.referenceKeys);
+    setCompletion(settings.i18n.completion);
+  };
+
+  const handleLanguagePreferenceChange = async (value: string) => {
+    setLanguage(value);
+    const response = await saveLanguagePreference(value);
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Arayüz dili kaydedildi.");
+    window.location.reload();
+  };
+
+  const handleCreateLocale = async () => {
+    const response = await createLocaleAction(newLocale);
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Dil eklendi.");
+    setSelectedLocale(response.locale?.code ?? newLocale.code);
+    await refreshI18nSettings();
+  };
+
+  const handleUpdateLocaleStatus = async (code: string, status: LocaleStatus) => {
+    const response = await updateLocaleStatusAction(code, status);
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Dil durumu güncellendi.");
+    await refreshI18nSettings();
+  };
+
+  const handleSetDefaultLocale = async (code: string) => {
+    const response = await setDefaultLocaleAction(code);
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Varsayılan dil güncellendi.");
+    await refreshI18nSettings();
+  };
+
+  const handleSaveTranslation = async (reference: ReferenceKey) => {
+    const value = editingValues[reference.key] ?? getTranslationValue(translations, selectedLocale, reference);
+    const response = await saveUiTranslationAction({
+      locale: selectedLocale,
+      namespace: reference.namespace,
+      key: reference.translationKey,
+      value,
+    });
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Çeviri kaydedildi.");
+    await refreshI18nSettings();
+  };
+
+  const handleResetTranslation = async (reference: ReferenceKey) => {
+    const response = await resetUiTranslationAction({
+      locale: selectedLocale,
+      namespace: reference.namespace,
+      key: reference.translationKey,
+    });
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Override sıfırlandı.");
+    setEditingValues((current) => ({ ...current, [reference.key]: "" }));
+    await refreshI18nSettings();
+  };
+
+  const handleExportI18n = async () => {
+    const response = await exportI18nAction();
+    if (response.error || !response.package) {
+      toast.error(response.error ?? "Export oluşturulamadı.");
+      return;
+    }
+    setImportJson(JSON.stringify(response.package, null, 2));
+    toast.success("Çeviri paketi aşağıdaki alana yazıldı.");
+  };
+
+  const handlePreviewImport = async () => {
+    const response = await previewI18nImportAction(importJson);
+    if (response.error || !response.preview) {
+      setImportPreview("");
+      toast.error(response.error ?? "Import paketi okunamadı.");
+      return;
+    }
+    setImportPreview(`${response.preview.localeCount} dil, ${response.preview.translationCount} çeviri, default: ${response.preview.defaultLocale}`);
+  };
+
+  const handleCommitImport = async () => {
+    const response = await commitI18nImportAction(importJson);
+    if (response.error) {
+      toast.error(response.error);
+      return;
+    }
+    toast.success("Çeviri paketi içe aktarıldı.");
+    setImportPreview("");
+    await refreshI18nSettings();
+  };
+
+  const filteredReferenceKeys = referenceKeys.filter((reference) => {
+    const namespaceMatches = selectedNamespace === "all" || reference.namespace === selectedNamespace;
+    const query = translationSearch.trim().toLowerCase();
+    const textMatches = !query || [reference.key, reference.tr, reference.en]
+      .some((value) => value.toLowerCase().includes(query));
+    return namespaceMatches && textMatches;
+  });
+
   return (
     <div className="mx-auto flex max-w-7xl flex-col gap-6">
       <div className="flex flex-col gap-4 border-b border-border pb-5 lg:flex-row lg:items-end lg:justify-between">
@@ -442,6 +639,22 @@ export default function SettingsPage() {
                     </div>
                   </section>
 
+                  <section className="space-y-4 border-t border-border pt-7">
+                    <div className="space-y-1">
+                      <h3 className="text-sm font-semibold text-foreground">Portal metinleri</h3>
+                      <p className="text-xs text-muted-foreground">
+                        Müşteri portalında kullanılacak karşılama ve footer metinlerini aktif dillere göre girin.
+                      </p>
+                    </div>
+                    <LocalizedFields
+                      idPrefix="branding-content"
+                      defaultLocale={defaultLocale}
+                      locales={locales.filter((locale) => locale.status !== "archived")}
+                      fields={contentTranslationRegistry.branding}
+                      values={brandingContentTranslations}
+                    />
+                  </section>
+
                   <div className="flex items-center gap-3 border-t border-border pt-6">
                     <Button variant="default" effect="shine" type="submit" loading={isSavingBranding} className="gap-2">
                       <Upload className="h-4 w-4" aria-hidden="true" />
@@ -518,6 +731,223 @@ export default function SettingsPage() {
             </Card>
           )}
 
+          {activeTab === "Diller ve çeviriler" && (
+            <Card className="animate-in fade-in duration-300">
+              <CardContent className="space-y-8 p-6 sm:p-8">
+                <div className="flex flex-col gap-4 border-b border-border pb-6 lg:flex-row lg:items-start lg:justify-between">
+                  <div className="space-y-1.5">
+                    <h2 className="text-xl font-bold text-foreground">Diller ve çeviriler</h2>
+                    <p className="max-w-2xl text-sm text-muted-foreground">
+                      Instance dillerini, kişisel arayüz dilini ve katalog override metinlerini yönetin.
+                    </p>
+                  </div>
+                  <div className="rounded-md border border-border bg-muted/30 px-3 py-2 text-xs text-muted-foreground">
+                    Catalog v{catalogVersion}
+                  </div>
+                </div>
+
+                <section className="grid gap-5 lg:grid-cols-[minmax(0,1fr)_320px]">
+                  <div className="space-y-4">
+                    <div className="flex items-center gap-2">
+                      <Globe2 className="h-4 w-4 text-muted-foreground" />
+                      <h3 className="text-sm font-semibold text-foreground">Aktif diller</h3>
+                    </div>
+                    <div className="grid gap-3 md:grid-cols-2">
+                      {locales.map((locale) => {
+                        const localeCompletion = completion.find((item) => item.locale === locale.code);
+                        return (
+                          <div key={locale.code} className="rounded-md border border-border bg-card p-4">
+                            <div className="flex items-start justify-between gap-3">
+                              <div>
+                                <div className="flex items-center gap-2">
+                                  <span className="font-semibold text-foreground">{locale.nativeName}</span>
+                                  <span className="rounded-md bg-muted px-2 py-0.5 text-xs text-muted-foreground">{locale.code}</span>
+                                </div>
+                                <p className="mt-1 text-xs text-muted-foreground">
+                                  {locale.name} · {locale.status} · fallback: {locale.fallbackLocale ?? "-"}
+                                </p>
+                              </div>
+                              {locale.builtIn ? (
+                                <span className="rounded-md bg-primary/10 px-2 py-0.5 text-xs text-primary">Built-in</span>
+                              ) : null}
+                            </div>
+                            <div className="mt-4 h-2 overflow-hidden rounded-full bg-muted">
+                              <div
+                                className="h-full rounded-full bg-primary"
+                                style={{ width: `${localeCompletion?.percent ?? 0}%` }}
+                              />
+                            </div>
+                            <div className="mt-2 flex items-center justify-between text-xs text-muted-foreground">
+                              <span>{localeCompletion?.percent ?? 0}% tamamlandı</span>
+                              <span>{localeCompletion?.translated ?? 0}/{localeCompletion?.total ?? 0}</span>
+                            </div>
+                            <div className="mt-4 flex flex-wrap gap-2">
+                              <Button effect="shine" type="button" size="sm" variant="secondary" onClick={() => setSelectedLocale(locale.code)}>
+                                Düzenle
+                              </Button>
+                              {locale.status !== "active" ? (
+                                <Button effect="shine" type="button" size="sm" variant="secondary" onClick={() => handleUpdateLocaleStatus(locale.code, "active")}>
+                                  Aktifleştir
+                                </Button>
+                              ) : null}
+                              {locale.status === "active" ? (
+                                <Button effect="shine" type="button" size="sm" variant="secondary" onClick={() => handleSetDefaultLocale(locale.code)} disabled={defaultLocale === locale.code}>
+                                  {defaultLocale === locale.code ? "Varsayılan" : "Default yap"}
+                                </Button>
+                              ) : null}
+                              {!locale.builtIn && locale.status !== "archived" ? (
+                                <Button effect="shine" type="button" size="sm" variant="secondary" onClick={() => handleUpdateLocaleStatus(locale.code, "archived")}>
+                                  Arşivle
+                                </Button>
+                              ) : null}
+                            </div>
+                          </div>
+                        );
+                      })}
+                    </div>
+                  </div>
+
+                  <div className="space-y-4 rounded-md border border-border bg-muted/20 p-4">
+                    <h3 className="text-sm font-semibold text-foreground">Yeni dil ekle</h3>
+                    <div className="grid gap-3">
+                      <Input value={newLocale.code} onChange={(event) => setNewLocale((current) => ({ ...current, code: event.target.value }))} placeholder="fr" />
+                      <Input value={newLocale.name} onChange={(event) => setNewLocale((current) => ({ ...current, name: event.target.value }))} placeholder="French" />
+                      <Input value={newLocale.nativeName} onChange={(event) => setNewLocale((current) => ({ ...current, nativeName: event.target.value }))} placeholder="Français" />
+                      <select
+                        value={newLocale.fallbackLocale}
+                        onChange={(event) => setNewLocale((current) => ({ ...current, fallbackLocale: event.target.value }))}
+                        className="h-10 rounded-md border border-input bg-background px-3 text-sm text-foreground"
+                      >
+                        {locales.filter((locale) => locale.status !== "archived").map((locale) => (
+                          <option key={locale.code} value={locale.code}>{locale.nativeName}</option>
+                        ))}
+                      </select>
+                      <select
+                        value={newLocale.textDirection}
+                        onChange={(event) => setNewLocale((current) => ({ ...current, textDirection: event.target.value as "ltr" | "rtl" }))}
+                        className="h-10 rounded-md border border-input bg-background px-3 text-sm text-foreground"
+                      >
+                        <option value="ltr">LTR</option>
+                        <option value="rtl">RTL</option>
+                      </select>
+                      <Button effect="shine" type="button" variant="default" className="gap-2" onClick={handleCreateLocale}>
+                        <Languages className="h-4 w-4" />
+                        Dili ekle
+                      </Button>
+                    </div>
+                  </div>
+                </section>
+
+                <section className="grid gap-5 border-t border-border pt-7 lg:grid-cols-2">
+                  <div className="space-y-2">
+                    <Label>Kişisel arayüz dili</Label>
+                    <select
+                      value={language}
+                      onChange={(event) => handleLanguagePreferenceChange(event.target.value)}
+                      className="h-10 w-full rounded-md border border-input bg-background px-3 text-sm text-foreground"
+                    >
+                      {locales.filter((locale) => locale.status === "active").map((locale) => (
+                        <option key={locale.code} value={locale.code}>{locale.nativeName}</option>
+                      ))}
+                    </select>
+                  </div>
+                  <div className="space-y-2">
+                    <Label>Çeviri hedef dili</Label>
+                    <select
+                      value={selectedLocale}
+                      onChange={(event) => setSelectedLocale(event.target.value)}
+                      className="h-10 w-full rounded-md border border-input bg-background px-3 text-sm text-foreground"
+                    >
+                      {locales.filter((locale) => locale.status !== "archived").map((locale) => (
+                        <option key={locale.code} value={locale.code}>{locale.nativeName}</option>
+                      ))}
+                    </select>
+                  </div>
+                </section>
+
+                <section className="space-y-4 border-t border-border pt-7">
+                  <div className="flex flex-col gap-3 lg:flex-row lg:items-end">
+                    <div className="space-y-2 lg:w-56">
+                      <Label>Namespace</Label>
+                      <select
+                        value={selectedNamespace}
+                        onChange={(event) => setSelectedNamespace(event.target.value)}
+                        className="h-10 w-full rounded-md border border-input bg-background px-3 text-sm text-foreground"
+                      >
+                        <option value="all">Tümü</option>
+                        {[...new Set(referenceKeys.map((reference) => reference.namespace))].map((namespace) => (
+                          <option key={namespace} value={namespace}>{namespace}</option>
+                        ))}
+                      </select>
+                    </div>
+                    <div className="flex-1 space-y-2">
+                      <Label>Eksik anahtar veya metin ara</Label>
+                      <Input value={translationSearch} onChange={(event) => setTranslationSearch(event.target.value)} placeholder="navigation.projects" />
+                    </div>
+                  </div>
+
+                  <div className="space-y-3">
+                    {filteredReferenceKeys.slice(0, 60).map((reference) => {
+                      const currentValue = getTranslationValue(translations, selectedLocale, reference);
+                      const draftValue = editingValues[reference.key] ?? currentValue;
+                      const isMissing = !currentValue;
+                      return (
+                        <div key={reference.key} className="grid gap-3 rounded-md border border-border p-4 lg:grid-cols-[minmax(0,1fr)_minmax(0,1fr)]">
+                          <div className="space-y-2">
+                            <div className="flex flex-wrap items-center gap-2">
+                              <code className="rounded-md bg-muted px-2 py-1 text-xs">{reference.key}</code>
+                              {isMissing ? <span className="rounded-md bg-destructive/10 px-2 py-1 text-xs text-destructive">Eksik</span> : null}
+                            </div>
+                            <p className="text-sm text-muted-foreground">TR: {reference.tr}</p>
+                            <p className="text-sm text-muted-foreground">EN: {reference.en}</p>
+                          </div>
+                          <div className="space-y-2">
+                            <Textarea
+                              value={draftValue}
+                              onChange={(event) => setEditingValues((current) => ({ ...current, [reference.key]: event.target.value }))}
+                              rows={3}
+                              placeholder={reference.en || reference.tr}
+                            />
+                            <div className="flex gap-2">
+                              <Button effect="shine" type="button" size="sm" variant="default" onClick={() => handleSaveTranslation(reference)}>
+                                Kaydet
+                              </Button>
+                              <Button effect="shine" type="button" size="sm" variant="secondary" onClick={() => handleResetTranslation(reference)}>
+                                Override sıfırla
+                              </Button>
+                            </div>
+                          </div>
+                        </div>
+                      );
+                    })}
+                  </div>
+                </section>
+
+                <section className="space-y-4 border-t border-border pt-7">
+                  <div className="flex flex-col gap-2 sm:flex-row sm:items-center sm:justify-between">
+                    <h3 className="text-sm font-semibold text-foreground">JSON import / export</h3>
+                    <Button effect="shine" type="button" variant="secondary" className="gap-2" onClick={handleExportI18n}>
+                      <Download className="h-4 w-4" />
+                      Export oluştur
+                    </Button>
+                  </div>
+                  <Textarea
+                    value={importJson}
+                    onChange={(event) => setImportJson(event.target.value)}
+                    rows={8}
+                    className="font-mono text-xs"
+                    placeholder='{"format":"neta-i18n","version":1,...}'
+                  />
+                  {importPreview ? <p className="text-sm text-muted-foreground">{importPreview}</p> : null}
+                  <div className="flex flex-wrap gap-2">
+                    <Button effect="shine" type="button" variant="secondary" onClick={handlePreviewImport}>Preview</Button>
+                    <Button effect="shine" type="button" variant="default" onClick={handleCommitImport}>Import et</Button>
+                  </div>
+                </section>
+              </CardContent>
+            </Card>
+          )}
+
           {activeTab === "Profile & Account" && (
             <Card className="animate-in fade-in duration-300">
               <CardContent className="p-6 sm:p-8">
@@ -753,3 +1183,24 @@ function BrandingAssetField({
     </div>
   );
 }
+
+function getTranslationValue(
+  translations: TranslationRow[],
+  locale: string,
+  reference: ReferenceKey,
+): string {
+  return translations.find(
+    (translation) =>
+      translation.locale === locale &&
+      translation.namespace === reference.namespace &&
+      translation.key === reference.translationKey,
+  )?.value ?? "";
+}
+
+function toLocalizedValues(rows: ContentTranslationRow[]) {
+  return rows.reduce<Record<string, Record<string, string>>>((result, row) => {
+    result[row.locale] = result[row.locale] ?? {};
+    result[row.locale][row.field] = row.value;
+    return result;
+  }, {});
+}
diff --git a/scripts/i18n-phase1-smoke.mjs b/scripts/i18n-phase1-smoke.mjs
new file mode 100644
index 0000000..c770d80
--- /dev/null
+++ b/scripts/i18n-phase1-smoke.mjs
@@ -0,0 +1,125 @@
+import assert from "node:assert/strict";
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+import Database from "better-sqlite3";
+
+const dataDir = path.join(process.cwd(), ".data", `i18n-phase1-smoke-${Date.now()}`);
+const databasePath = path.join(dataDir, "neta.db");
+const restoreDir = `${dataDir}-restore`;
+const env = { ...process.env, DATA_DIR: dataDir, DATABASE_PATH: databasePath };
+
+fs.mkdirSync(dataDir, { recursive: true });
+
+execFileSync(process.execPath, ["scripts/migrate.mjs"], {
+  cwd: process.cwd(),
+  env,
+  stdio: "inherit",
+});
+
+assertMigratedDatabase(databasePath, "tr");
+
+execFileSync("./node_modules/.bin/tsc", ["-p", "tsconfig.i18n-phase1-smoke.json"], {
+  cwd: process.cwd(),
+  stdio: "inherit",
+});
+execFileSync(
+  process.execPath,
+  [path.join(".next", "i18n-phase1-smoke-dist", "scripts", "i18n-phase1-smoke.js"), databasePath],
+  { cwd: process.cwd(), stdio: "inherit" },
+);
+
+execFileSync(process.execPath, ["scripts/backup.mjs", "--retention-count", "1"], {
+  cwd: process.cwd(),
+  env,
+  stdio: "inherit",
+});
+
+const backupDir = latestBackupDir(path.join(dataDir, "backups"));
+execFileSync(
+  process.execPath,
+  ["scripts/restore.mjs", "--from", backupDir, "--target", restoreDir],
+  { cwd: process.cwd(), env: process.env, stdio: "inherit" },
+);
+
+assertMigratedDatabase(path.join(restoreDir, "neta.db"), "fr");
+assertRestoredI18nData(path.join(restoreDir, "neta.db"));
+
+console.log("I18n phase 1 migration, service and backup/restore smoke passed.");
+
+function assertMigratedDatabase(dbPath, expectedDefaultLocale) {
+  const sqlite = new Database(dbPath);
+  try {
+    for (const tableName of [
+      "instance_locales",
+      "instance_i18n_settings",
+      "instance_ui_translations",
+      "content_translations",
+    ]) {
+      assert.equal(
+        sqlite.prepare("select count(*) as value from sqlite_master where type = 'table' and name = ?").get(tableName).value,
+        1,
+        `${tableName} must exist`,
+      );
+    }
+
+    const builtInLocales = sqlite
+      .prepare("select code, status, fallback_locale as fallbackLocale, built_in as builtIn from instance_locales where code in ('tr', 'en') order by sort_order")
+      .all();
+    assert.deepEqual(
+      builtInLocales,
+      [
+        { code: "tr", status: "active", fallbackLocale: null, builtIn: 1 },
+        { code: "en", status: "active", fallbackLocale: "tr", builtIn: 1 },
+      ],
+      "Built-in locales must be seeded",
+    );
+    assert.deepEqual(
+      sqlite.prepare("select key, default_locale as defaultLocale, catalog_version as catalogVersion from instance_i18n_settings").all(),
+      [{ key: "default", defaultLocale: expectedDefaultLocale, catalogVersion: 1 }],
+      "Default i18n settings must be seeded",
+    );
+
+    const clientColumns = sqlite.prepare("pragma table_info(clients)").all().map((column) => column.name);
+    assert.equal(clientColumns.includes("portal_locale"), true, "clients.portal_locale must exist");
+    const invitationColumns = sqlite.prepare("pragma table_info(portal_invitations)").all().map((column) => column.name);
+    assert.equal(invitationColumns.includes("locale"), true, "portal_invitations.locale must exist");
+
+    sqlite.prepare(
+      "insert into user (id, name, email, email_verified, created_at, updated_at) values (?, ?, ?, ?, ?, ?)",
+    ).run("locale-preference-user", "Locale Preference User", "locale-preference@example.com", 1, Date.now(), Date.now());
+    sqlite.prepare("insert into user_preferences (owner_user_id, language) values (?, ?)").run("locale-preference-user", "fr");
+    sqlite.prepare("delete from user_preferences where owner_user_id = ?").run("locale-preference-user");
+    sqlite.prepare("delete from user where id = ?").run("locale-preference-user");
+  } finally {
+    sqlite.close();
+  }
+}
+
+function assertRestoredI18nData(dbPath) {
+  const sqlite = new Database(dbPath);
+  try {
+    assert.equal(
+      sqlite.prepare("select default_locale as value from instance_i18n_settings where key = 'default'").get().value,
+      "fr",
+      "Restored backup must include updated i18n settings",
+    );
+    assert.equal(
+      sqlite.prepare("select value from content_translations where locale = 'fr' and entity_id = 'i18n-project'").get().value,
+      "Projet multilingue",
+      "Restored backup must include content translations",
+    );
+  } finally {
+    sqlite.close();
+  }
+}
+
+function latestBackupDir(backupsDir) {
+  const entries = fs
+    .readdirSync(backupsDir, { withFileTypes: true })
+    .filter((entry) => entry.isDirectory())
+    .map((entry) => path.join(backupsDir, entry.name))
+    .sort((left, right) => path.basename(right).localeCompare(path.basename(left)));
+  assert.ok(entries[0], "Backup directory must contain a backup");
+  return entries[0];
+}
diff --git a/scripts/i18n-phase1-smoke.ts b/scripts/i18n-phase1-smoke.ts
new file mode 100644
index 0000000..b9c21d0
--- /dev/null
+++ b/scripts/i18n-phase1-smoke.ts
@@ -0,0 +1,132 @@
+import assert from "node:assert/strict";
+import Database from "better-sqlite3";
+import { drizzle } from "drizzle-orm/better-sqlite3";
+import * as schema from "../server/db/schema";
+import type { DomainActor } from "../server/domain/actor";
+import { DomainError } from "../server/domain/errors";
+import { I18nService } from "../server/i18n/service";
+
+const databasePath = process.argv[2];
+assert.ok(databasePath, "Database path is required");
+
+const sqlite = new Database(databasePath);
+sqlite.pragma("foreign_keys = ON");
+const db = drizzle({ client: sqlite, schema });
+
+const owner: DomainActor = {
+  authUserId: "i18n-owner",
+  role: "freelancer",
+  clientId: null,
+  disabled: false,
+};
+const client: DomainActor = {
+  authUserId: "i18n-client-user",
+  role: "client",
+  clientId: "i18n-client",
+  disabled: false,
+};
+const disabledOwner: DomainActor = {
+  ...owner,
+  authUserId: "i18n-disabled-owner",
+  disabled: true,
+};
+
+try {
+  for (const actor of [owner, client, disabledOwner]) {
+    db.insert(schema.user).values({
+      id: actor.authUserId,
+      name: actor.authUserId,
+      email: `${actor.authUserId}@example.com`,
+      emailVerified: true,
+      createdAt: new Date(),
+      updatedAt: new Date(),
+    }).run();
+  }
+
+  const service = new I18nService(db);
+  const locales = service.listLocales(owner);
+  assert.deepEqual(
+    locales.map((locale) => ({
+      code: locale.code,
+      status: locale.status,
+      builtIn: locale.builtIn,
+      fallbackLocale: locale.fallbackLocale,
+    })),
+    [
+      { code: "tr", status: "active", builtIn: true, fallbackLocale: null },
+      { code: "en", status: "active", builtIn: true, fallbackLocale: "tr" },
+    ],
+  );
+  assert.deepEqual(service.getSettings(owner), { defaultLocale: "tr", catalogVersion: 1 });
+
+  assertDomainError(() => service.listLocales(client), "FORBIDDEN");
+  assertDomainError(() => service.createLocale(disabledOwner, { code: "fr", name: "French" }), "FORBIDDEN");
+  assertDomainError(() => service.createLocale(owner, { code: "english", name: "English" }), "VALIDATION_ERROR");
+  assertDomainError(() => service.createLocale(owner, { code: "en", name: "English" }), "CONFLICT");
+  assertDomainError(
+    () => service.createLocale(owner, { code: "de", name: "German", fallbackLocale: "de" }),
+    "VALIDATION_ERROR",
+  );
+  assertDomainError(
+    () => service.createLocale(owner, { code: "pt", name: "Portuguese", fallbackLocale: "zz" }),
+    "VALIDATION_ERROR",
+  );
+
+  const french = service.createLocale(owner, {
+    code: "fr",
+    name: "French",
+    nativeName: "Français",
+    fallbackLocale: "en",
+  });
+  assert.equal(french.status, "draft");
+  assert.equal(french.fallbackLocale, "en");
+
+  service.createLocale(owner, { code: "es", name: "Spanish", fallbackLocale: "fr" });
+  assertDomainError(() => service.updateLocale(owner, "fr", { fallbackLocale: "es" }), "VALIDATION_ERROR");
+  assertDomainError(() => service.setDefaultLocale(owner, "fr"), "VALIDATION_ERROR");
+
+  const activeFrench = service.updateLocale(owner, "fr", { status: "active" });
+  assert.equal(activeFrench.status, "active");
+  assert.equal(service.setDefaultLocale(owner, "fr").defaultLocale, "fr");
+  assertDomainError(() => service.archiveLocale(owner, "fr"), "CONFLICT");
+  assertDomainError(() => service.archiveLocale(owner, "en"), "CONFLICT");
+
+  service.createLocale(owner, {
+    code: "it",
+    name: "Italian",
+    nativeName: "Italiano",
+    status: "active",
+    fallbackLocale: "en",
+  });
+  db.insert(schema.clients).values({
+    id: "i18n-client",
+    ownerUserId: owner.authUserId,
+    name: "I18n Client",
+    authUserId: client.authUserId,
+    portalLocale: "it",
+  }).run();
+  assertDomainError(() => service.archiveLocale(owner, "it"), "CONFLICT");
+
+  db.insert(schema.contentTranslations).values({
+    entityType: "project",
+    entityId: "i18n-project",
+    field: "name",
+    locale: "fr",
+    value: "Projet multilingue",
+  }).run();
+
+  const persisted = db.select().from(schema.instanceLocales).all();
+  assert.equal(persisted.some((locale) => locale.code === "fr" && locale.status === "active"), true);
+  assert.equal(
+    db.select().from(schema.contentTranslations).all()[0]?.value,
+    "Projet multilingue",
+  );
+
+  console.log("I18n phase 1 service smoke passed.");
+} finally {
+  sqlite.close();
+}
+
+function assertDomainError(run: () => unknown, code: DomainError["code"]): void {
+  assert.throws(run, (error) => error instanceof DomainError && error.code === code);
+}
diff --git a/scripts/i18n-phase2-smoke.mjs b/scripts/i18n-phase2-smoke.mjs
new file mode 100644
index 0000000..828a617
--- /dev/null
+++ b/scripts/i18n-phase2-smoke.mjs
@@ -0,0 +1,27 @@
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+
+const dataDir = path.join(process.cwd(), ".data", `i18n-phase2-smoke-${Date.now()}`);
+const databasePath = path.join(dataDir, "neta.db");
+const env = { ...process.env, DATA_DIR: dataDir, DATABASE_PATH: databasePath };
+
+fs.mkdirSync(dataDir, { recursive: true });
+
+execFileSync(process.execPath, ["scripts/migrate.mjs"], {
+  cwd: process.cwd(),
+  env,
+  stdio: "inherit",
+});
+execFileSync("./node_modules/.bin/tsc", ["-p", "tsconfig.i18n-phase2-smoke.json"], {
+  cwd: process.cwd(),
+  stdio: "inherit",
+});
+const serverOnlyStubDir = path.join(process.cwd(), ".next", "i18n-phase2-smoke-dist", "node_modules", "server-only");
+fs.mkdirSync(serverOnlyStubDir, { recursive: true });
+fs.writeFileSync(path.join(serverOnlyStubDir, "index.js"), "\n");
+execFileSync(
+  process.execPath,
+  [path.join(".next", "i18n-phase2-smoke-dist", "scripts", "i18n-phase2-smoke.js")],
+  { cwd: process.cwd(), env, stdio: "inherit" },
+);
diff --git a/scripts/i18n-phase2-smoke.ts b/scripts/i18n-phase2-smoke.ts
new file mode 100644
index 0000000..3371a4a
--- /dev/null
+++ b/scripts/i18n-phase2-smoke.ts
@@ -0,0 +1,93 @@
+import assert from "node:assert/strict";
+import {
+  compareCatalogKeys,
+  createTranslatorFromMessages,
+  formatDate,
+  formatMoney,
+  I18N_NAMESPACES,
+} from "../lib/i18n";
+import { enCatalog } from "../locales/en";
+import { trCatalog } from "../locales/tr";
+import { getSqliteConnection } from "../server/db/client";
+import type { DomainActor } from "../server/domain/actor";
+import { DomainError } from "../server/domain/errors";
+import { I18nService } from "../server/i18n/service";
+import { directionForLocale } from "../server/i18n/locale";
+import { createTranslator } from "../server/i18n/translator";
+
+const owner: DomainActor = {
+  authUserId: "i18n-phase2-owner",
+  role: "freelancer",
+  clientId: null,
+  disabled: false,
+};
+const client: DomainActor = {
+  authUserId: "i18n-phase2-client",
+  role: "client",
+  clientId: "i18n-phase2-client",
+  disabled: false,
+};
+
+const parity = compareCatalogKeys(trCatalog, enCatalog, I18N_NAMESPACES);
+assert.deepEqual(parity, { missingInLeft: [], missingInRight: [] }, "TR/EN catalog keys must match");
+
+const clientTranslator = createTranslatorFromMessages("en", {
+  "common.greeting": "Hello {name}",
+  "common.items": "{count, plural, one {# item} other {# items}}",
+});
+assert.equal(clientTranslator.t("common.greeting", { name: "Neta" }), "Hello Neta");
+assert.equal(clientTranslator.t("common.items", { count: 1 }), "1 item");
+assert.equal(clientTranslator.t("common.items", { count: 3 }), "3 items");
+assert.equal(directionForLocale("ar-XB"), "rtl");
+assert.match(formatDate("2026-07-19", "en"), /July/);
+assert.match(formatMoney(12345, "USD", "en"), /\$123\.45/);
+
+const service = new I18nService(getSqliteConnection().db);
+service.listLocales(owner);
+
+const tr = createTranslator("tr", ["common"]);
+const en = createTranslator("en", ["common"]);
+assert.equal(tr.t("common.actions.save"), "Kaydet");
+assert.equal(en.t("common.actions.save"), "Save");
+
+service.createLocale(owner, {
+  code: "fr",
+  name: "French",
+  nativeName: "Français",
+  fallbackLocale: "en",
+});
+service.upsertUiTranslation(owner, {
+  locale: "fr",
+  namespace: "common",
+  key: "actions.save",
+  value: "Enregistrer",
+});
+assertDomainError(
+  () => service.upsertUiTranslation(client, {
+    locale: "fr",
+    namespace: "common",
+    key: "actions.cancel",
+    value: "Annuler",
+  }),
+  "FORBIDDEN",
+);
+
+const fr = createTranslator("fr", ["common"]);
+assert.equal(fr.t("common.actions.save"), "Enregistrer");
+assert.equal(fr.t("common.actions.cancel"), "Cancel");
+assert.equal(fr.t("common.missing.key"), "common.missing.key");
+
+service.upsertUiTranslation(owner, {
+  locale: "fr",
+  namespace: "common",
+  key: "actions.cancel",
+  value: "Annuler",
+});
+const frAfterBump = createTranslator("fr", ["common"]);
+assert.equal(frAfterBump.t("common.actions.cancel"), "Annuler");
+
+console.log("I18n phase 2 runtime smoke passed.");
+
+function assertDomainError(run: () => unknown, code: DomainError["code"]): void {
+  assert.throws(run, (error) => error instanceof DomainError && error.code === code);
+}
diff --git a/scripts/i18n-phase3-smoke.mjs b/scripts/i18n-phase3-smoke.mjs
new file mode 100644
index 0000000..1c2f282
--- /dev/null
+++ b/scripts/i18n-phase3-smoke.mjs
@@ -0,0 +1,27 @@
+import { execFileSync } from "node:child_process";
+import fs from "node:fs";
+import path from "node:path";
+
+const dataDir = path.join(process.cwd(), ".data", `i18n-phase3-smoke-${Date.now()}`);
+const databasePath = path.join(dataDir, "neta.db");
+const env = { ...process.env, DATA_DIR: dataDir, DATABASE_PATH: databasePath };
+
+fs.mkdirSync(dataDir, { recursive: true });
+
+execFileSync(process.execPath, ["scripts/migrate.mjs"], {
+  cwd: process.cwd(),
+  env,
+  stdio: "inherit",
+});
+execFileSync("./node_modules/.bin/tsc", ["-p", "tsconfig.i18n-phase3-smoke.json"], {
+  cwd: process.cwd(),
+  stdio: "inherit",
+});
+const serverOnlyStubDir = path.join(process.cwd(), ".next", "i18n-phase3-smoke-dist", "node_modules", "server-only");
+fs.mkdirSync(serverOnlyStubDir, { recursive: true });
+fs.writeFileSync(path.join(serverOnlyStubDir, "index.js"), "\n");
+execFileSync(
+  process.execPath,
+  [path.join(".next", "i18n-phase3-smoke-dist", "scripts", "i18n-phase3-smoke.js")],
+  { cwd: process.cwd(), env, stdio: "inherit" },
+);
diff --git a/scripts/i18n-phase3-smoke.ts b/scripts/i18n-phase3-smoke.ts
new file mode 100644
index 0000000..deb9c27
--- /dev/null
+++ b/scripts/i18n-phase3-smoke.ts
@@ -0,0 +1,72 @@
+import assert from "node:assert/strict";
+import { getSqliteConnection } from "../server/db/client";
+import type { DomainActor } from "../server/domain/actor";
+import { DomainError } from "../server/domain/errors";
+import { I18nService } from "../server/i18n/service";
+
+const owner: DomainActor = {
+  authUserId: "phase3-owner",
+  role: "freelancer",
+  clientId: null,
+  disabled: false,
+};
+const client: DomainActor = {
+  authUserId: "phase3-client-user",
+  role: "client",
+  clientId: "phase3-client",
+  disabled: false,
+};
+
+const service = new I18nService(getSqliteConnection().db);
+service.listLocales(owner);
+
+assertDomainError(() => service.listLocales(client), "FORBIDDEN");
+
+const fr = service.createLocale(owner, {
+  code: "fr",
+  name: "French",
+  nativeName: "Français",
+  fallbackLocale: "en",
+});
+assert.equal(fr.status, "draft");
+
+const beforeCompletion = service.getCompletion(owner).find((item) => item.locale === "fr");
+assert.ok(beforeCompletion);
+assert.ok(beforeCompletion.missingKeys.includes("navigation.items.projects"));
+
+service.upsertUiTranslation(owner, {
+  locale: "fr",
+  namespace: "navigation",
+  key: "items.projects",
+  value: "Projets",
+});
+
+const exported = service.exportPackage(owner);
+assert.equal(exported.format, "neta-i18n");
+assert.equal(exported.translations.some((row) => row.locale === "fr" && row.value === "Projets"), true);
+
+service.resetUiTranslation(owner, {
+  locale: "fr",
+  namespace: "navigation",
+  key: "items.projects",
+});
+assert.equal(
+  service.listUiTranslations(owner).some((row) => row.locale === "fr" && row.value === "Projets"),
+  false,
+);
+
+service.importPackage(owner, exported);
+assert.equal(
+  service.listUiTranslations(owner).some((row) => row.locale === "fr" && row.value === "Projets"),
+  true,
+);
+
+service.updateLocale(owner, "fr", { status: "active" });
+assert.equal(service.setDefaultLocale(owner, "fr").defaultLocale, "fr");
+assertDomainError(() => service.archiveLocale(owner, "fr"), "CONFLICT");
+
+console.log("I18n phase 3 settings smoke passed.");
+
+function assertDomainError(run: () => unknown, code: DomainError["code"]): void {
+  assert.throws(run, (error) => error instanceof DomainError && error.code === code);
+}
diff --git a/server/settings/preferences.ts b/server/settings/preferences.ts
index 43e50b8..9702458 100644
--- a/server/settings/preferences.ts
+++ b/server/settings/preferences.ts
@@ -11,22 +11,30 @@ import { DomainError } from "@/server/domain/errors";
 const colorModeInputSchema = z.object({
   colorMode: z.enum(["light", "dark", "system"]),
 });
+const languageInputSchema = z.object({
+  language: z.string().trim().regex(/^[a-z]{2}(?:-[A-Z]{2}[0-9]?)?$/),
+});
 
 export type PublicUserPreferences = {
   colorMode: ColorMode;
+  language: string;
 };
 
 export function getUserPreferences(actor: DomainActor): PublicUserPreferences {
   assertEnabledActor(actor);
 
   const row = getSqliteConnection().db
-    .select({ colorMode: userPreferences.colorMode })
+    .select({
+      colorMode: userPreferences.colorMode,
+      language: userPreferences.language,
+    })
     .from(userPreferences)
     .where(eq(userPreferences.ownerUserId, actor.authUserId))
     .get();
 
   return {
     colorMode: (row?.colorMode as ColorMode | undefined) ?? "system",
+    language: row?.language ?? "tr",
   };
 }
 
@@ -56,5 +64,41 @@ export function updateColorModePreference(
     })
     .run();
 
-  return { colorMode: parsed.data.colorMode };
+  return {
+    colorMode: parsed.data.colorMode,
+    language: getUserPreferences(actor).language,
+  };
+}
+
+export function updateLanguagePreference(
+  actor: DomainActor,
+  input: unknown,
+): PublicUserPreferences {
+  assertEnabledActor(actor);
+
+  const parsed = languageInputSchema.safeParse(input);
+  if (!parsed.success) {
+    throw new DomainError("VALIDATION_ERROR", "Dil tercihi geçersiz.");
+  }
+
+  const { db } = getSqliteConnection();
+  db.insert(userPreferences)
+    .values({
+      ownerUserId: actor.authUserId,
+      language: parsed.data.language,
+    })
+    .onConflictDoUpdate({
+      target: userPreferences.ownerUserId,
+      set: {
+        language: parsed.data.language,
+        updatedAt: new Date().toISOString(),
+      },
+    })
+    .run();
+
+  const preferences = getUserPreferences(actor);
+  return {
+    ...preferences,
+    language: parsed.data.language,
+  };
 }
diff --git a/tsconfig.i18n-phase1-smoke.json b/tsconfig.i18n-phase1-smoke.json
new file mode 100644
index 0000000..f418642
--- /dev/null
+++ b/tsconfig.i18n-phase1-smoke.json
@@ -0,0 +1,16 @@
+{
+  "extends": "./tsconfig.phase2-smoke.json",
+  "compilerOptions": {
+    "outDir": ".next/i18n-phase1-smoke-dist"
+  },
+  "include": [
+    "scripts/i18n-phase1-smoke.ts",
+    "server/auth/types.ts",
+    "server/db/schema/**/*.ts",
+    "server/domain/**/*.ts",
+    "server/i18n/locale.ts",
+    "server/i18n/service.ts",
+    "server/repositories/i18n.ts"
+  ],
+  "exclude": ["node_modules"]
+}
diff --git a/tsconfig.i18n-phase2-smoke.json b/tsconfig.i18n-phase2-smoke.json
new file mode 100644
index 0000000..9113101
--- /dev/null
+++ b/tsconfig.i18n-phase2-smoke.json
@@ -0,0 +1,21 @@
+{
+  "extends": "./tsconfig.phase2-smoke.json",
+  "compilerOptions": {
+    "outDir": ".next/i18n-phase2-smoke-dist"
+  },
+  "include": [
+    "scripts/i18n-phase2-smoke.ts",
+    "components/i18n/**/*.tsx",
+    "lib/i18n/**/*.ts",
+    "locales/**/*.ts",
+    "server/auth/types.ts",
+    "server/db/**/*.ts",
+    "server/domain/**/*.ts",
+    "server/i18n/catalog.ts",
+    "server/i18n/locale.ts",
+    "server/i18n/service.ts",
+    "server/i18n/translator.ts",
+    "server/repositories/i18n.ts"
+  ],
+  "exclude": ["node_modules"]
+}
diff --git a/tsconfig.i18n-phase3-smoke.json b/tsconfig.i18n-phase3-smoke.json
new file mode 100644
index 0000000..da22f55
--- /dev/null
+++ b/tsconfig.i18n-phase3-smoke.json
@@ -0,0 +1,20 @@
+{
+  "extends": "./tsconfig.i18n-phase2-smoke.json",
+  "compilerOptions": {
+    "outDir": ".next/i18n-phase3-smoke-dist"
+  },
+  "include": [
+    "scripts/i18n-phase3-smoke.ts",
+    "lib/i18n/**/*.ts",
+    "locales/**/*.ts",
+    "server/auth/types.ts",
+    "server/db/**/*.ts",
+    "server/domain/**/*.ts",
+    "server/i18n/catalog.ts",
+    "server/i18n/locale.ts",
+    "server/i18n/service.ts",
+    "server/i18n/translator.ts",
+    "server/repositories/i18n.ts"
+  ],
+  "exclude": ["node_modules"]
+}