diff --git a/app/api/i18n/locale/route.ts b/app/api/i18n/locale/route.ts
new file mode 100644
index 0000000..6125f5f
--- /dev/null
+++ b/app/api/i18n/locale/route.ts
@@ -0,0 +1,53 @@
+import { cookies } from "next/headers";
+import { NextResponse } from "next/server";
+import { getSqliteConnection } from "@/server/db/client";
+import { instanceLocales } from "@/server/db/schema";
+import { DomainError } from "@/server/domain/errors";
+import { buildLocaleCookie, normalizeLocaleCode } from "@/server/i18n/locale";
+import { eq } from "drizzle-orm";
+
+export const runtime = "nodejs";
+export const dynamic = "force-dynamic";
+
+export async function POST(request: Request) {
+  try {
+    const payload = await request.json();
+    const locale = normalizeLocaleCode(typeof payload.locale === "string" ? payload.locale : null);
+    if (!locale) {
+      throw new DomainError("VALIDATION_ERROR", "validation.invalidLocale", {
+        messageKey: "validation.invalidLocale",
+      });
+    }
+
+    const row = getSqliteConnection().db
+      .select({ code: instanceLocales.code, status: instanceLocales.status })
+      .from(instanceLocales)
+      .where(eq(instanceLocales.code, locale))
+      .get();
+    if (!row || row.status === "archived") {
+      throw new DomainError("UNSUPPORTED_LOCALE", "validation.unsupportedLocale", {
+        messageKey: "validation.unsupportedLocale",
+      });
+    }
+
+    (await cookies()).set(buildLocaleCookie(row.code));
+    return NextResponse.json({ ok: true, data: { locale: row.code } });
+  } catch (error) {
+    const normalized = error instanceof DomainError
+      ? error
+      : new DomainError("VALIDATION_ERROR", "validation.unsupportedLocale", {
+        messageKey: "validation.unsupportedLocale",
+      });
+    return NextResponse.json(
+      {
+        ok: false,
+        error: {
+          code: normalized.code,
+          message: normalized.message,
+          messageKey: normalized.details?.messageKey,
+        },
+      },
+      { status: normalized.status },
+    );
+  }
+}
diff --git a/components/i18n/i18n-provider.tsx b/components/i18n/i18n-provider.tsx
new file mode 100644
index 0000000..76399b0
--- /dev/null
+++ b/components/i18n/i18n-provider.tsx
@@ -0,0 +1,51 @@
+"use client";
+
+import {
+  createContext,
+  useContext,
+  useMemo,
+  type ReactNode,
+} from "react";
+import {
+  createTranslatorFromMessages,
+  type Translator,
+  type TranslationValues,
+} from "@/lib/i18n";
+
+type I18nContextValue = Translator;
+
+const I18nContext = createContext<I18nContextValue | null>(null);
+
+export function I18nProvider({
+  children,
+  locale,
+  messages,
+}: {
+  children: ReactNode;
+  locale: string;
+  messages: Record<string, string>;
+}) {
+  const value = useMemo(
+    () => createTranslatorFromMessages(locale, messages),
+    [locale, messages],
+  );
+
+  return <I18nContext.Provider value={value}>{children}</I18nContext.Provider>;
+}
+
+export function useTranslations() {
+  const value = useContext(I18nContext);
+  if (!value) {
+    throw new Error("useTranslations must be used within I18nProvider.");
+  }
+
+  return (key: string, values?: TranslationValues) => value.t(key, values);
+}
+
+export function useI18n() {
+  const value = useContext(I18nContext);
+  if (!value) {
+    throw new Error("useI18n must be used within I18nProvider.");
+  }
+  return value;
+}
diff --git a/components/i18n/locale-select-form.tsx b/components/i18n/locale-select-form.tsx
new file mode 100644
index 0000000..887b119
--- /dev/null
+++ b/components/i18n/locale-select-form.tsx
@@ -0,0 +1,58 @@
+"use client";
+
+import { useState, useTransition } from "react";
+import { Label } from "poyraz-ui/atoms";
+import {
+  Select,
+  SelectContent,
+  SelectItem,
+  SelectTrigger,
+  SelectValue,
+} from "poyraz-ui/molecules";
+
+type LocaleOption = {
+  code: string;
+  nativeName: string;
+  name: string;
+};
+
+type LocaleSelectFormProps = {
+  label: string;
+  value: string;
+  locales: LocaleOption[];
+};
+
+export function LocaleSelectForm({ label, value, locales }: LocaleSelectFormProps) {
+  const [currentLocale, setCurrentLocale] = useState(value);
+  const [pending, startTransition] = useTransition();
+
+  function handleChange(locale: string) {
+    setCurrentLocale(locale);
+    startTransition(async () => {
+      await fetch("/api/i18n/locale", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ locale }),
+      });
+      window.location.reload();
+    });
+  }
+
+  return (
+    <div className="space-y-2">
+      <Label htmlFor="auth-locale">{label}</Label>
+      <Select value={currentLocale} onValueChange={handleChange} disabled={pending}>
+        <SelectTrigger id="auth-locale" aria-label={label}>
+          <SelectValue />
+        </SelectTrigger>
+        <SelectContent>
+          {locales.map((locale) => (
+            <SelectItem key={locale.code} value={locale.code}>
+              {locale.nativeName || locale.name}
+            </SelectItem>
+          ))}
+        </SelectContent>
+      </Select>
+    </div>
+  );
+}
diff --git a/components/i18n/localized-fields.tsx b/components/i18n/localized-fields.tsx
new file mode 100644
index 0000000..47e42db
--- /dev/null
+++ b/components/i18n/localized-fields.tsx
@@ -0,0 +1,117 @@
+"use client";
+
+import { useMemo, useState } from "react";
+import { Badge, Button, Input, Label, Textarea } from "poyraz-ui/atoms";
+import { contentInputName, type ContentTranslationField } from "@/lib/i18n/content";
+
+export type LocalizedFieldLocale = {
+  code: string;
+  nativeName: string;
+  status?: string;
+};
+
+export type LocalizedFieldValues = Record<string, Record<string, string | null | undefined>>;
+
+type LocalizedFieldsProps = {
+  idPrefix: string;
+  defaultLocale: string;
+  locales: LocalizedFieldLocale[];
+  fields: ContentTranslationField[];
+  values?: LocalizedFieldValues | null;
+  fallbackValues?: Record<string, string | null | undefined>;
+};
+
+export function LocalizedFields({
+  idPrefix,
+  defaultLocale,
+  locales,
+  fields,
+  values,
+  fallbackValues,
+}: LocalizedFieldsProps) {
+  const orderedLocales = useMemo(() => {
+    const localeMap = new Map(locales.map((locale) => [locale.code, locale]));
+    const defaultLocaleRecord = localeMap.get(defaultLocale);
+    return [
+      ...(defaultLocaleRecord ? [defaultLocaleRecord] : []),
+      ...locales.filter((locale) => locale.code !== defaultLocale),
+    ];
+  }, [defaultLocale, locales]);
+  const [activeLocale, setActiveLocale] = useState(orderedLocales[0]?.code ?? defaultLocale);
+  const missingRequiredLocales = new Set(
+    orderedLocales
+      .filter((locale) =>
+        fields.some((field) => {
+          if (!field.required) return false;
+          const value = values?.[locale.code]?.[field.name] ?? (
+            locale.code === defaultLocale ? fallbackValues?.[field.name] : null
+          );
+          return !String(value ?? "").trim();
+        }),
+      )
+      .map((locale) => locale.code),
+  );
+
+  return (
+    <div className="grid gap-4 rounded-sm border border-border bg-muted/10 p-3">
+      <div className="tiny-scrollbar flex gap-2 overflow-x-auto pb-1">
+        {orderedLocales.map((locale) => (
+          <Button
+            key={locale.code}
+            type="button"
+            variant={activeLocale === locale.code ? "default" : "secondary"}
+            effect="shine"
+            size="sm"
+            className="shrink-0 gap-2"
+            onClick={() => setActiveLocale(locale.code)}
+          >
+            {locale.nativeName || locale.code}
+            {locale.code === defaultLocale ? <Badge variant="secondary">varsayılan</Badge> : null}
+            {missingRequiredLocales.has(locale.code) ? <span aria-hidden="true" className="text-amber-500">•</span> : null}
+          </Button>
+        ))}
+      </div>
+
+      {orderedLocales.map((locale) => (
+        <div key={locale.code} className={activeLocale === locale.code ? "grid gap-4" : "hidden"}>
+          {fields.map((field) => {
+            const id = `${idPrefix}-${locale.code}-${field.name}`;
+            const defaultValue =
+              values?.[locale.code]?.[field.name] ??
+              (locale.code === defaultLocale ? fallbackValues?.[field.name] : "") ??
+              "";
+
+            return (
+              <div key={field.name} className="grid gap-2">
+                <Label htmlFor={id}>
+                  {field.label}
+                  {field.required && locale.code === defaultLocale ? <span className="text-destructive"> *</span> : null}
+                </Label>
+                {field.kind === "textarea" ? (
+                  <Textarea
+                    id={id}
+                    name={contentInputName(locale.code, field.name)}
+                    defaultValue={String(defaultValue ?? "")}
+                    maxLength={field.maxLength}
+                    required={field.required && locale.code === defaultLocale}
+                    placeholder={field.placeholder}
+                    rows={field.name === "content" ? 8 : 3}
+                  />
+                ) : (
+                  <Input
+                    id={id}
+                    name={contentInputName(locale.code, field.name)}
+                    defaultValue={String(defaultValue ?? "")}
+                    maxLength={field.maxLength}
+                    required={field.required && locale.code === defaultLocale}
+                    placeholder={field.placeholder}
+                  />
+                )}
+              </div>
+            );
+          })}
+        </div>
+      ))}
+    </div>
+  );
+}
diff --git a/server/i18n/catalog.ts b/server/i18n/catalog.ts
new file mode 100644
index 0000000..783ce88
--- /dev/null
+++ b/server/i18n/catalog.ts
@@ -0,0 +1,92 @@
+import { cache } from "react";
+import {
+  DEFAULT_LOCALE,
+  flattenCatalog,
+  getBuiltInCatalog,
+  type I18nNamespace,
+} from "../../lib/i18n";
+import { getSqliteConnection } from "../db/client";
+import { instanceI18nSettings } from "../db/schema";
+import { createI18nRepository } from "../repositories/i18n";
+
+export type ResolvedCatalog = {
+  locale: string;
+  requestedLocale: string;
+  namespaces: I18nNamespace[];
+  catalogVersion: number;
+  fallbackChain: string[];
+  messages: Record<string, string>;
+};
+
+export const getResolvedCatalog = cache(
+  (
+    requestedLocale: string,
+    namespaces: readonly I18nNamespace[],
+    catalogVersion?: number,
+  ): ResolvedCatalog => {
+    const { db } = getSqliteConnection();
+    const repository = createI18nRepository(db);
+    const version = catalogVersion ?? ensureCatalogVersion(db);
+    const fallbackChain = resolveFallbackChain(repository, requestedLocale);
+    const messages: Record<string, string> = {};
+
+    for (const locale of [...fallbackChain].reverse()) {
+      const builtIn = getBuiltInCatalog(locale);
+      if (builtIn) {
+        Object.assign(messages, flattenCatalog(builtIn, namespaces));
+      }
+
+      for (const row of repository.listUiTranslations(locale, namespaces)) {
+        messages[`${row.namespace}.${row.translationKey}`] = row.value;
+      }
+    }
+
+    return {
+      locale: fallbackChain[0] ?? DEFAULT_LOCALE,
+      requestedLocale,
+      namespaces: [...namespaces],
+      catalogVersion: version,
+      fallbackChain,
+      messages,
+    };
+  },
+);
+
+export function getCatalogVersion(): number {
+  return ensureCatalogVersion(getSqliteConnection().db);
+}
+
+function ensureCatalogVersion(db: ReturnType<typeof getSqliteConnection>["db"]): number {
+  const repository = createI18nRepository(db);
+  repository.createSettingsIfMissing();
+  return db.select({ catalogVersion: instanceI18nSettings.catalogVersion }).from(instanceI18nSettings).get()
+    ?.catalogVersion ?? 1;
+}
+
+function resolveFallbackChain(
+  repository: ReturnType<typeof createI18nRepository>,
+  requestedLocale: string,
+): string[] {
+  const locales = new Map(repository.listLocales().map((locale) => [locale.code, locale]));
+  const settings = repository.getSettings();
+  const start = locales.has(requestedLocale)
+    ? requestedLocale
+    : settings?.defaultLocale ?? DEFAULT_LOCALE;
+  const chain: string[] = [];
+  const seen = new Set<string>();
+  let cursor: string | null = start;
+
+  while (cursor && !seen.has(cursor)) {
+    const locale = locales.get(cursor);
+    if (!locale || locale.status === "archived") break;
+    chain.push(cursor);
+    seen.add(cursor);
+    cursor = locale.fallbackLocale;
+  }
+
+  if (!chain.includes(DEFAULT_LOCALE)) {
+    chain.push(DEFAULT_LOCALE);
+  }
+
+  return chain;
+}
diff --git a/server/i18n/content.ts b/server/i18n/content.ts
new file mode 100644
index 0000000..f04dd27
--- /dev/null
+++ b/server/i18n/content.ts
@@ -0,0 +1,275 @@
+import "server-only";
+
+import { and, eq, inArray } from "drizzle-orm";
+import {
+  contentInputName,
+  contentTranslationRegistry,
+  type ContentTranslationInput,
+} from "../../lib/i18n/content";
+import {
+  contentTranslations,
+  instanceI18nSettings,
+  instanceLocales,
+  type TranslationEntityType,
+} from "../db/schema";
+import type { DomainActor } from "../domain/actor";
+import type { DomainDatabase } from "../domain/database";
+import { DomainError } from "../domain/errors";
+import { I18nService, type LocaleRecord } from "./service";
+
+export type ContentTranslationRow = {
+  entityType: TranslationEntityType;
+  entityId: string;
+  field: string;
+  locale: string;
+  value: string;
+};
+
+export type ContentLocalizationContext = {
+  defaultLocale: string;
+  locales: LocaleRecord[];
+};
+
+export class ContentTranslationService {
+  constructor(private readonly db: DomainDatabase) {}
+
+  getLocalizationContext(actor: DomainActor): ContentLocalizationContext {
+    const i18n = new I18nService(this.db);
+    const settings = i18n.getSettings(actor);
+    const locales = i18n
+      .listLocales(actor)
+      .filter((locale) => locale.status !== "archived")
+      .sort((a, b) => a.sortOrder - b.sortOrder || a.code.localeCompare(b.code));
+
+    return {
+      defaultLocale: settings.defaultLocale,
+      locales,
+    };
+  }
+
+  getPublicLocalizationContext(): ContentLocalizationContext {
+    const settings = this.db
+      .select()
+      .from(instanceI18nSettings)
+      .where(eq(instanceI18nSettings.key, "default"))
+      .get();
+    const locales = this.db
+      .select()
+      .from(instanceLocales)
+      .where(eq(instanceLocales.status, "active"))
+      .orderBy(instanceLocales.sortOrder, instanceLocales.code)
+      .all()
+      .map(toContentLocaleRecord);
+
+    return {
+      defaultLocale: settings?.defaultLocale ?? "tr",
+      locales,
+    };
+  }
+
+  listEntityTranslations(entityType: TranslationEntityType, entityId: string): ContentTranslationRow[] {
+    return this.db
+      .select()
+      .from(contentTranslations)
+      .where(and(eq(contentTranslations.entityType, entityType), eq(contentTranslations.entityId, entityId)))
+      .all()
+      .map(toContentTranslationRow);
+  }
+
+  listBatch(entityType: TranslationEntityType, entityIds: readonly string[]): Map<string, ContentTranslationRow[]> {
+    const uniqueIds = [...new Set(entityIds)].filter(Boolean);
+    const result = new Map<string, ContentTranslationRow[]>();
+    if (uniqueIds.length === 0) return result;
+
+    for (const row of this.db
+      .select()
+      .from(contentTranslations)
+      .where(and(eq(contentTranslations.entityType, entityType), inArray(contentTranslations.entityId, uniqueIds)))
+      .all()
+      .map(toContentTranslationRow)) {
+      const rows = result.get(row.entityId) ?? [];
+      rows.push(row);
+      result.set(row.entityId, rows);
+    }
+
+    return result;
+  }
+
+  upsertEntityTranslations(
+    entityType: TranslationEntityType,
+    entityId: string,
+    input: ContentTranslationInput | null | undefined,
+  ) {
+    const fields = new Set(contentTranslationRegistry[entityType].map((field) => field.name));
+    if (!input) return;
+
+    for (const [locale, values] of Object.entries(input)) {
+      for (const [field, rawValue] of Object.entries(values)) {
+        if (!fields.has(field)) continue;
+        const value = normalizeOptionalText(rawValue);
+
+        if (!value) {
+          this.db
+            .delete(contentTranslations)
+            .where(
+              and(
+                eq(contentTranslations.entityType, entityType),
+                eq(contentTranslations.entityId, entityId),
+                eq(contentTranslations.field, field),
+                eq(contentTranslations.locale, locale),
+              ),
+            )
+            .run();
+          continue;
+        }
+
+        this.db
+          .insert(contentTranslations)
+          .values({ entityType, entityId, field, locale, value })
+          .onConflictDoUpdate({
+            target: [
+              contentTranslations.entityType,
+              contentTranslations.entityId,
+              contentTranslations.field,
+              contentTranslations.locale,
+            ],
+            set: {
+              value,
+              updatedAt: new Date(),
+            },
+          })
+          .run();
+      }
+    }
+  }
+
+  deleteEntityTranslations(entityType: TranslationEntityType, entityId: string) {
+    this.db
+      .delete(contentTranslations)
+      .where(and(eq(contentTranslations.entityType, entityType), eq(contentTranslations.entityId, entityId)))
+      .run();
+  }
+
+  resolveEntity<T extends Record<string, unknown>>(
+    entityType: TranslationEntityType,
+    entity: T,
+    options: {
+      locale: string;
+      fallbackLocale?: string | null;
+      defaultLocale: string;
+      translations?: ContentTranslationRow[];
+    },
+  ): T {
+    const translations = options.translations ?? this.listEntityTranslations(entityType, String(entity.id ?? ""));
+    const resolved = { ...entity };
+
+    for (const field of contentTranslationRegistry[entityType]) {
+      const value = resolveFieldValue(translations, field.name, [
+        options.locale,
+        options.fallbackLocale,
+        options.defaultLocale,
+      ]);
+      if (value != null) {
+        (resolved as Record<string, unknown>)[field.name] = value;
+      }
+    }
+
+    return resolved;
+  }
+}
+
+export function parseContentTranslationsFromFormData(
+  formData: FormData,
+  entityType: TranslationEntityType,
+  context: ContentLocalizationContext,
+): ContentTranslationInput {
+  const fields = contentTranslationRegistry[entityType];
+  const translations: ContentTranslationInput = {};
+
+  for (const locale of context.locales) {
+    const values: Record<string, string | null> = {};
+    for (const field of fields) {
+      const raw = formData.get(contentInputName(locale.code, field.name));
+      values[field.name] = normalizeOptionalText(typeof raw === "string" ? raw : null);
+    }
+    translations[locale.code] = values;
+  }
+
+  assertDefaultLocaleRequiredFields(entityType, translations, context.defaultLocale);
+  return translations;
+}
+
+export function projectBaseFromTranslations<T extends Record<string, unknown>>(
+  entityType: TranslationEntityType,
+  payload: T,
+  translations: ContentTranslationInput,
+  defaultLocale: string,
+): T {
+  const next = { ...payload };
+  const defaultValues = translations[defaultLocale] ?? {};
+
+  for (const field of contentTranslationRegistry[entityType]) {
+    const value = normalizeOptionalText(defaultValues[field.name]);
+    if (value != null) {
+      (next as Record<string, unknown>)[field.name] = value;
+    }
+  }
+
+  return next;
+}
+
+export function getContentFallbackLocale(
+  locale: string,
+  context: ContentLocalizationContext,
+): string | null {
+  return context.locales.find((item) => item.code === locale)?.fallbackLocale ?? null;
+}
+
+function assertDefaultLocaleRequiredFields(
+  entityType: TranslationEntityType,
+  translations: ContentTranslationInput,
+  defaultLocale: string,
+) {
+  const defaultValues = translations[defaultLocale] ?? {};
+
+  for (const field of contentTranslationRegistry[entityType]) {
+    if (!("required" in field) || !field.required) continue;
+    if (!normalizeOptionalText(defaultValues[field.name])) {
+      throw new DomainError("VALIDATION_ERROR", `${field.label} varsayılan dilde zorunludur.`);
+    }
+  }
+}
+
+function resolveFieldValue(rows: ContentTranslationRow[], field: string, localeChain: Array<string | null | undefined>) {
+  const uniqueLocales = [...new Set(localeChain.filter(Boolean) as string[])];
+  return rows.find((row) => row.field === field && uniqueLocales.includes(row.locale))?.value ?? null;
+}
+
+function normalizeOptionalText(value: string | null | undefined) {
+  if (typeof value !== "string") return null;
+  const normalized = value.trim();
+  return normalized.length > 0 ? normalized : null;
+}
+
+function toContentTranslationRow(row: typeof contentTranslations.$inferSelect): ContentTranslationRow {
+  return {
+    entityType: row.entityType,
+    entityId: row.entityId,
+    field: row.field,
+    locale: row.locale,
+    value: row.value,
+  };
+}
+
+function toContentLocaleRecord(row: typeof instanceLocales.$inferSelect): LocaleRecord {
+  return {
+    code: row.code,
+    name: row.name,
+    nativeName: row.nativeName,
+    status: row.status,
+    fallbackLocale: row.fallbackLocale,
+    textDirection: row.textDirection,
+    builtIn: row.builtIn,
+    sortOrder: row.sortOrder,
+  };
+}
diff --git a/server/i18n/locale.ts b/server/i18n/locale.ts
new file mode 100644
index 0000000..f9ba070
--- /dev/null
+++ b/server/i18n/locale.ts
@@ -0,0 +1,28 @@
+import type { TextDirection } from "../../lib/i18n";
+
+export const LOCALE_COOKIE = "neta_locale";
+export const LOCALE_COOKIE_MAX_AGE = 60 * 60 * 24 * 365;
+export const DEFAULT_TEXT_DIRECTION: TextDirection = "ltr";
+
+export function normalizeLocaleCode(value: string | null | undefined): string | null {
+  if (!value) return null;
+  const normalized = value.trim();
+  return /^[a-z]{2}(?:-[A-Z]{2}[0-9]?)?$/.test(normalized) ? normalized : null;
+}
+
+export function directionForLocale(locale: string, explicit?: TextDirection | null): TextDirection {
+  if (explicit) return explicit;
+  return /^(ar|fa|he|ur)(-|$)/.test(locale) ? "rtl" : DEFAULT_TEXT_DIRECTION;
+}
+
+export function buildLocaleCookie(value: string) {
+  return {
+    name: LOCALE_COOKIE,
+    value,
+    maxAge: LOCALE_COOKIE_MAX_AGE,
+    sameSite: "lax" as const,
+    path: "/",
+    httpOnly: false,
+    secure: process.env.NODE_ENV === "production",
+  };
+}
diff --git a/server/i18n/resolver.ts b/server/i18n/resolver.ts
new file mode 100644
index 0000000..d435269
--- /dev/null
+++ b/server/i18n/resolver.ts
@@ -0,0 +1,82 @@
+import { cookies, headers } from "next/headers";
+import { cache } from "react";
+import { DEFAULT_LOCALE } from "../../lib/i18n";
+import { getSessionContextFromHeaders } from "../auth/session";
+import { getSqliteConnection } from "../db/client";
+import { clients, instanceI18nSettings, instanceLocales, userPreferences } from "../db/schema";
+import { eq } from "drizzle-orm";
+import { directionForLocale, LOCALE_COOKIE, normalizeLocaleCode } from "./locale";
+
+export type ResolvedLocale = {
+  locale: string;
+  requestedLocale: string | null;
+  defaultLocale: string;
+  direction: "ltr" | "rtl";
+  source: "user" | "client" | "cookie" | "instance" | "fallback";
+};
+
+export const resolveRequestLocale = cache(async (): Promise<ResolvedLocale> => {
+  const requestHeaders = await headers();
+  const cookieStore = await cookies();
+  const context = await getSessionContextFromHeaders(requestHeaders);
+  const { db } = getSqliteConnection();
+
+  const settings = db.select().from(instanceI18nSettings).where(eq(instanceI18nSettings.key, "default")).get();
+  const defaultLocale = settings?.defaultLocale ?? DEFAULT_LOCALE;
+  const cookieLocale = normalizeLocaleCode(cookieStore.get(LOCALE_COOKIE)?.value);
+  let requestedLocale: string | null = null;
+  let source: ResolvedLocale["source"] = "fallback";
+
+  if (context?.profile.role === "client" && context.profile.clientId) {
+    requestedLocale = normalizeLocaleCode(
+      db
+        .select({ portalLocale: clients.portalLocale })
+        .from(clients)
+        .where(eq(clients.id, context.profile.clientId))
+        .get()?.portalLocale,
+    );
+    source = requestedLocale ? "client" : source;
+  }
+
+  if (!requestedLocale && context) {
+    requestedLocale = normalizeLocaleCode(
+      db
+        .select({ language: userPreferences.language })
+        .from(userPreferences)
+        .where(eq(userPreferences.ownerUserId, context.profile.authUserId))
+        .get()?.language,
+    );
+    source = requestedLocale ? "user" : source;
+  }
+
+  if (!requestedLocale && cookieLocale) {
+    requestedLocale = cookieLocale;
+    source = "cookie";
+  }
+
+  if (!requestedLocale) {
+    requestedLocale = defaultLocale;
+    source = "instance";
+  }
+
+  const localeRow = db
+    .select({
+      code: instanceLocales.code,
+      status: instanceLocales.status,
+      textDirection: instanceLocales.textDirection,
+    })
+    .from(instanceLocales)
+    .where(eq(instanceLocales.code, requestedLocale))
+    .get();
+  const locale = localeRow && localeRow.status !== "archived"
+    ? localeRow.code
+    : defaultLocale || DEFAULT_LOCALE;
+
+  return {
+    locale,
+    requestedLocale,
+    defaultLocale,
+    direction: directionForLocale(locale, localeRow?.textDirection),
+    source,
+  };
+});
diff --git a/server/i18n/runtime.ts b/server/i18n/runtime.ts
new file mode 100644
index 0000000..8c11474
--- /dev/null
+++ b/server/i18n/runtime.ts
@@ -0,0 +1,28 @@
+import { getSqliteConnection } from "../db/client";
+import { createI18nRepository } from "../repositories/i18n";
+
+export function getPublicLocalizationMetadata() {
+  const repository = createI18nRepository(getSqliteConnection().db);
+  repository.createSettingsIfMissing();
+  const settings = repository.getSettings();
+  const locales = repository.listLocales().filter((locale) => locale.status !== "archived");
+
+  return {
+    defaultLocale: settings?.defaultLocale ?? "tr",
+    supportedLocales: locales.map((locale) => ({
+      code: locale.code,
+      name: locale.name,
+      nativeName: locale.nativeName,
+      status: locale.status,
+      fallbackLocale: locale.fallbackLocale,
+      textDirection: locale.textDirection,
+      builtIn: locale.builtIn,
+    })),
+    fallbacks: Object.fromEntries(
+      locales
+        .filter((locale) => locale.fallbackLocale)
+        .map((locale) => [locale.code, locale.fallbackLocale]),
+    ),
+    catalogVersion: settings?.catalogVersion ?? 1,
+  };
+}
diff --git a/server/i18n/service.ts b/server/i18n/service.ts
new file mode 100644
index 0000000..9d8caaf
--- /dev/null
+++ b/server/i18n/service.ts
@@ -0,0 +1,504 @@
+import type { LocaleStatus, TextDirection } from "../db/schema";
+import {
+  DEFAULT_LOCALE,
+  I18N_NAMESPACES,
+  compareCatalogKeys,
+  flattenCatalog,
+  getBuiltInCatalog,
+  type I18nNamespace,
+} from "../../lib/i18n";
+import { trCatalog } from "../../locales/tr";
+import { requireOwnerScope, type DomainActor } from "../domain/actor";
+import type { DomainDatabase } from "../domain/database";
+import { DomainError, notFound } from "../domain/errors";
+import { createI18nRepository } from "../repositories/i18n";
+
+export type LocaleRecord = {
+  code: string;
+  name: string;
+  nativeName: string;
+  status: LocaleStatus;
+  fallbackLocale: string | null;
+  textDirection: TextDirection;
+  builtIn: boolean;
+  sortOrder: number;
+};
+
+export type I18nSettingsRecord = {
+  defaultLocale: string;
+  catalogVersion: number;
+};
+
+export type CreateLocaleInput = {
+  code: string;
+  name: string;
+  nativeName?: string;
+  fallbackLocale?: string | null;
+  textDirection?: TextDirection;
+  status?: LocaleStatus;
+  sortOrder?: number;
+};
+
+export type UpdateLocaleInput = Partial<Omit<CreateLocaleInput, "code">>;
+
+export type UpsertUiTranslationInput = {
+  locale: string;
+  namespace: string;
+  key: string;
+  value: string;
+};
+
+export type UiTranslationRow = {
+  locale: string;
+  namespace: string;
+  key: string;
+  value: string;
+};
+
+export type I18nExportPackage = {
+  format: "neta-i18n";
+  version: 1;
+  exportedAt: string;
+  defaultLocale: string;
+  locales: LocaleRecord[];
+  translations: UiTranslationRow[];
+};
+
+export type TranslationCompletion = {
+  locale: string;
+  translated: number;
+  total: number;
+  percent: number;
+  missingKeys: string[];
+};
+
+const LOCALE_CODE_PATTERN = /^[a-z]{2}(?:-[A-Z]{2}[0-9]?)?$/;
+const BUILT_IN_LOCALES = new Set(["tr", "en"]);
+
+export class I18nService {
+  private readonly repository;
+
+  constructor(private readonly db: DomainDatabase) {
+    this.repository = createI18nRepository(db);
+  }
+
+  listLocales(actor: DomainActor): LocaleRecord[] {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+    return this.repository.listLocales().map(toLocaleRecord);
+  }
+
+  getSettings(actor: DomainActor): I18nSettingsRecord {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+    return toSettingsRecord(this.repository.getSettings());
+  }
+
+  createLocale(actor: DomainActor, input: CreateLocaleInput): LocaleRecord {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const code = normalizeLocaleCode(input.code);
+    if (BUILT_IN_LOCALES.has(code)) {
+      throw new DomainError("CONFLICT", "Built-in dil zaten mevcut.");
+    }
+    if (this.repository.getLocale(code)) {
+      throw new DomainError("CONFLICT", "Bu dil zaten eklenmiş.");
+    }
+
+    const fallbackLocale = normalizeOptionalLocale(input.fallbackLocale ?? "en");
+    this.assertValidFallback(code, fallbackLocale);
+
+    const created = this.repository.createLocale({
+      code,
+      name: normalizeRequiredText(input.name, "Dil adı zorunludur."),
+      nativeName: normalizeRequiredText(input.nativeName ?? input.name, "Yerel dil adı zorunludur."),
+      fallbackLocale,
+      textDirection: input.textDirection ?? "ltr",
+      status: input.status ?? "draft",
+      builtIn: false,
+      sortOrder: input.sortOrder ?? 100,
+    });
+
+    return toLocaleRecord(created);
+  }
+
+  updateLocale(actor: DomainActor, code: string, input: UpdateLocaleInput): LocaleRecord {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const locale = this.getExistingLocale(code);
+    if (locale.builtIn && input.status === "archived") {
+      throw new DomainError("CONFLICT", "Built-in diller arşivlenemez.");
+    }
+
+    const nextFallback = input.fallbackLocale === undefined
+      ? locale.fallbackLocale
+      : normalizeOptionalLocale(input.fallbackLocale);
+    this.assertValidFallback(locale.code, nextFallback);
+
+    const nextStatus = input.status ?? locale.status;
+    if (nextStatus === "archived") {
+      this.assertLocaleCanBeArchived(locale.code);
+    }
+
+    const updated = this.repository.updateLocale(locale.code, {
+      name: input.name === undefined ? undefined : normalizeRequiredText(input.name, "Dil adı zorunludur."),
+      nativeName: input.nativeName === undefined
+        ? undefined
+        : normalizeRequiredText(input.nativeName, "Yerel dil adı zorunludur."),
+      fallbackLocale: nextFallback,
+      textDirection: input.textDirection,
+      status: nextStatus,
+      sortOrder: input.sortOrder,
+    });
+
+    if (!updated) throw notFound("Dil");
+    return toLocaleRecord(updated);
+  }
+
+  archiveLocale(actor: DomainActor, code: string): LocaleRecord {
+    return this.updateLocale(actor, code, { status: "archived" });
+  }
+
+  setDefaultLocale(actor: DomainActor, code: string): I18nSettingsRecord {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const locale = this.getExistingLocale(code);
+    if (locale.status !== "active") {
+      throw new DomainError("VALIDATION_ERROR", "Varsayılan dil yalnızca aktif bir dil olabilir.");
+    }
+
+    const updated = this.repository.updateDefaultLocale(locale.code);
+    return toSettingsRecord(updated);
+  }
+
+  upsertUiTranslation(actor: DomainActor, input: UpsertUiTranslationInput): void {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const locale = this.getExistingLocale(input.locale);
+    if (locale.status === "archived") {
+      throw new DomainError("VALIDATION_ERROR", "Arşivlenmiş dile çeviri yazılamaz.");
+    }
+
+    const namespace = normalizeIdentifier(input.namespace, "Namespace geçersiz.", 64);
+    const translationKey = normalizeIdentifier(input.key, "Çeviri anahtarı geçersiz.", 160);
+    const value = input.value.trim();
+    if (!value) {
+      throw new DomainError("VALIDATION_ERROR", "Çeviri metni boş olamaz.");
+    }
+
+    this.repository.upsertUiTranslation({
+      locale: locale.code,
+      namespace,
+      translationKey,
+      value,
+    });
+    this.repository.bumpCatalogVersion();
+  }
+
+  resetUiTranslation(actor: DomainActor, input: Omit<UpsertUiTranslationInput, "value">): void {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const locale = this.getExistingLocale(input.locale);
+    this.repository.deleteUiTranslation(
+      locale.code,
+      normalizeIdentifier(input.namespace, "Namespace geçersiz.", 64),
+      normalizeIdentifier(input.key, "Çeviri anahtarı geçersiz.", 160),
+    );
+    this.repository.bumpCatalogVersion();
+  }
+
+  listUiTranslations(actor: DomainActor): UiTranslationRow[] {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+    return this.repository.listAllUiTranslations().map((row) => ({
+      locale: row.locale,
+      namespace: row.namespace,
+      key: row.translationKey,
+      value: row.value,
+    }));
+  }
+
+  getCompletion(actor: DomainActor): TranslationCompletion[] {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const referenceKeys = Object.keys(flattenCatalog(trCatalog, I18N_NAMESPACES));
+    const overrideRows = this.repository.listAllUiTranslations();
+    const overrideKeysByLocale = new Map<string, Set<string>>();
+    for (const row of overrideRows) {
+      const keys = overrideKeysByLocale.get(row.locale) ?? new Set<string>();
+      keys.add(`${row.namespace}.${row.translationKey}`);
+      overrideKeysByLocale.set(row.locale, keys);
+    }
+
+    return this.repository.listLocales().map((locale) => {
+      const builtIn = getBuiltInCatalog(locale.code);
+      const builtInKeys = builtIn ? new Set(Object.keys(flattenCatalog(builtIn, I18N_NAMESPACES))) : new Set<string>();
+      const overrideKeys = overrideKeysByLocale.get(locale.code) ?? new Set<string>();
+      const missingKeys = referenceKeys.filter((key) => !builtInKeys.has(key) && !overrideKeys.has(key));
+      const translated = referenceKeys.length - missingKeys.length;
+      return {
+        locale: locale.code,
+        translated,
+        total: referenceKeys.length,
+        percent: referenceKeys.length ? Math.round((translated / referenceKeys.length) * 100) : 100,
+        missingKeys,
+      };
+    });
+  }
+
+  exportPackage(actor: DomainActor): I18nExportPackage {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    return {
+      format: "neta-i18n",
+      version: 1,
+      exportedAt: new Date().toISOString(),
+      defaultLocale: this.getSettings(actor).defaultLocale,
+      locales: this.listLocales(actor),
+      translations: this.listUiTranslations(actor),
+    };
+  }
+
+  importPackage(actor: DomainActor, input: unknown): I18nExportPackage {
+    requireOwnerScope(actor);
+    this.ensureBootstrap();
+
+    const parsed = parseImportPackage(input);
+    for (const locale of parsed.locales) {
+      if (!this.repository.getLocale(locale.code) && !BUILT_IN_LOCALES.has(locale.code)) {
+        this.createLocale(actor, {
+          code: locale.code,
+          name: locale.name,
+          nativeName: locale.nativeName,
+          fallbackLocale: locale.fallbackLocale ?? DEFAULT_LOCALE,
+          textDirection: locale.textDirection,
+          status: locale.status === "archived" ? "draft" : locale.status,
+          sortOrder: locale.sortOrder,
+        });
+      }
+    }
+
+    for (const translation of parsed.translations) {
+      this.upsertUiTranslation(actor, translation);
+    }
+
+    const defaultLocale = parsed.defaultLocale ? this.repository.getLocale(parsed.defaultLocale) : null;
+    if (defaultLocale?.status === "active") {
+      this.setDefaultLocale(actor, defaultLocale.code);
+    }
+
+    return this.exportPackage(actor);
+  }
+
+  private ensureBootstrap(): void {
+    this.repository.createSettingsIfMissing();
+    ensureBuiltInLocale(this.repository, {
+      code: "tr",
+      name: "Turkish",
+      nativeName: "Türkçe",
+      status: "active",
+      fallbackLocale: null,
+      textDirection: "ltr",
+      builtIn: true,
+      sortOrder: 10,
+    });
+    ensureBuiltInLocale(this.repository, {
+      code: "en",
+      name: "English",
+      nativeName: "English",
+      status: "active",
+      fallbackLocale: "tr",
+      textDirection: "ltr",
+      builtIn: true,
+      sortOrder: 20,
+    });
+  }
+
+  private getExistingLocale(code: string) {
+    const normalized = normalizeLocaleCode(code);
+    const locale = this.repository.getLocale(normalized);
+    if (!locale) throw notFound("Dil");
+    return locale;
+  }
+
+  private assertValidFallback(code: string, fallbackLocale: string | null): void {
+    if (!fallbackLocale) return;
+    if (fallbackLocale === code) {
+      throw new DomainError("VALIDATION_ERROR", "Dil kendi kendine fallback olamaz.");
+    }
+
+    const fallback = this.repository.getLocale(fallbackLocale);
+    if (!fallback || fallback.status === "archived") {
+      throw new DomainError("VALIDATION_ERROR", "Fallback dili aktif veya taslak bir dil olmalıdır.");
+    }
+
+    const graph = new Map(
+      this.repository
+        .listLocales()
+        .map((locale) => [locale.code, locale.fallbackLocale] as const),
+    );
+    graph.set(code, fallbackLocale);
+
+    const seen = new Set<string>();
+    let cursor: string | null = fallbackLocale;
+    while (cursor) {
+      if (cursor === code || seen.has(cursor)) {
+        throw new DomainError("VALIDATION_ERROR", "Fallback zinciri döngü oluşturamaz.");
+      }
+      seen.add(cursor);
+      cursor = graph.get(cursor) ?? null;
+    }
+  }
+
+  private assertLocaleCanBeArchived(code: string): void {
+    const references = this.repository.countLocaleReferences(code);
+    const referenceCount = Object.values(references).reduce((total, value) => total + value, 0);
+    if (referenceCount > 0) {
+      throw new DomainError("CONFLICT", "Kullanımda olan dil arşivlenemez.", references);
+    }
+  }
+}
+
+export function getReferenceTranslationKeys(namespace: I18nNamespace | "all" = "all") {
+  const namespaces = namespace === "all" ? I18N_NAMESPACES : [namespace];
+  const tr = flattenCatalog(trCatalog, namespaces);
+  const en = flattenCatalog(getBuiltInCatalog("en") ?? trCatalog, namespaces);
+  const parity = compareCatalogKeys(trCatalog, getBuiltInCatalog("en") ?? trCatalog, I18N_NAMESPACES);
+  return Object.keys(tr).sort().map((key) => ({
+    key,
+    namespace: key.split(".")[0] as I18nNamespace,
+    translationKey: key.split(".").slice(1).join("."),
+    tr: tr[key] ?? "",
+    en: en[key] ?? "",
+    parityOk: parity.missingInLeft.length === 0 && parity.missingInRight.length === 0,
+  }));
+}
+
+function ensureBuiltInLocale(
+  repository: ReturnType<typeof createI18nRepository>,
+  value: Parameters<ReturnType<typeof createI18nRepository>["createLocale"]>[0],
+): void {
+  const existing = repository.getLocale(value.code);
+  if (!existing) {
+    repository.createLocale(value);
+    return;
+  }
+
+  repository.updateLocale(value.code, {
+    name: value.name,
+    nativeName: value.nativeName,
+    status: value.status,
+    fallbackLocale: value.fallbackLocale,
+    textDirection: value.textDirection,
+    builtIn: true,
+    sortOrder: value.sortOrder,
+  });
+}
+
+function normalizeLocaleCode(code: string): string {
+  const normalized = code.trim();
+  if (!LOCALE_CODE_PATTERN.test(normalized)) {
+    throw new DomainError("VALIDATION_ERROR", "Dil kodu BCP47 kısa formatında olmalıdır. Örn: tr, en, fr veya ar-XB.");
+  }
+  return normalized;
+}
+
+function normalizeOptionalLocale(code: string | null | undefined): string | null {
+  if (!code) return null;
+  return normalizeLocaleCode(code);
+}
+
+function normalizeRequiredText(value: string, message: string): string {
+  const normalized = value.trim();
+  if (!normalized) throw new DomainError("VALIDATION_ERROR", message);
+  if (normalized.length > 80) {
+    throw new DomainError("VALIDATION_ERROR", "Dil adı 80 karakterden kısa olmalıdır.");
+  }
+  return normalized;
+}
+
+function normalizeIdentifier(value: string, message: string, maxLength: number): string {
+  const normalized = value.trim();
+  if (!normalized || normalized.length > maxLength || !/^[a-zA-Z0-9_.-]+$/.test(normalized)) {
+    throw new DomainError("VALIDATION_ERROR", message);
+  }
+  return normalized;
+}
+
+function toLocaleRecord(value: {
+  code: string;
+  name: string;
+  nativeName: string;
+  status: LocaleStatus;
+  fallbackLocale: string | null;
+  textDirection: TextDirection;
+  builtIn: boolean;
+  sortOrder: number;
+}): LocaleRecord {
+  return {
+    code: value.code,
+    name: value.name,
+    nativeName: value.nativeName,
+    status: value.status,
+    fallbackLocale: value.fallbackLocale,
+    textDirection: value.textDirection,
+    builtIn: value.builtIn,
+    sortOrder: value.sortOrder,
+  };
+}
+
+function toSettingsRecord(value: {
+  defaultLocale: string;
+  catalogVersion: number;
+} | undefined): I18nSettingsRecord {
+  if (!value) {
+    throw new DomainError("INVARIANT_VIOLATION", "I18n ayarları oluşturulamadı.");
+  }
+  return {
+    defaultLocale: value.defaultLocale,
+    catalogVersion: value.catalogVersion,
+  };
+}
+
+function parseImportPackage(input: unknown): I18nExportPackage {
+  if (!input || typeof input !== "object") {
+    throw new DomainError("VALIDATION_ERROR", "Import paketi geçersiz.");
+  }
+  const value = input as Partial<I18nExportPackage>;
+  if (value.format !== "neta-i18n" || value.version !== 1) {
+    throw new DomainError("VALIDATION_ERROR", "Import paketi desteklenmiyor.");
+  }
+  if (!Array.isArray(value.locales) || !Array.isArray(value.translations)) {
+    throw new DomainError("VALIDATION_ERROR", "Import paketinde locale veya çeviri listesi eksik.");
+  }
+
+  return {
+    format: "neta-i18n",
+    version: 1,
+    exportedAt: typeof value.exportedAt === "string" ? value.exportedAt : new Date().toISOString(),
+    defaultLocale: typeof value.defaultLocale === "string" ? value.defaultLocale : DEFAULT_LOCALE,
+    locales: value.locales.map((locale) => ({
+      code: normalizeLocaleCode(locale.code),
+      name: normalizeRequiredText(locale.name, "Dil adı zorunludur."),
+      nativeName: normalizeRequiredText(locale.nativeName, "Yerel dil adı zorunludur."),
+      status: locale.status,
+      fallbackLocale: normalizeOptionalLocale(locale.fallbackLocale),
+      textDirection: locale.textDirection === "rtl" ? "rtl" : "ltr",
+      builtIn: Boolean(locale.builtIn),
+      sortOrder: Number.isFinite(locale.sortOrder) ? locale.sortOrder : 100,
+    })),
+    translations: value.translations.map((translation) => ({
+      locale: normalizeLocaleCode(translation.locale),
+      namespace: normalizeIdentifier(translation.namespace, "Namespace geçersiz.", 64),
+      key: normalizeIdentifier(translation.key, "Çeviri anahtarı geçersiz.", 160),
+      value: normalizeRequiredText(translation.value, "Çeviri metni boş olamaz."),
+    })),
+  };
+}
diff --git a/server/i18n/translator.ts b/server/i18n/translator.ts
new file mode 100644
index 0000000..4633e5b
--- /dev/null
+++ b/server/i18n/translator.ts
@@ -0,0 +1,42 @@
+import {
+  createTranslatorFromMessages,
+  type I18nNamespace,
+  type TranslationValues,
+} from "../../lib/i18n";
+import { getCatalogVersion, getResolvedCatalog } from "./catalog";
+
+export type ServerTranslator = {
+  locale: string;
+  fallbackChain: string[];
+  messages: Record<string, string>;
+  t: (key: string, values?: TranslationValues) => string;
+};
+
+export function createTranslator(
+  locale: string,
+  namespaces: readonly I18nNamespace[],
+): ServerTranslator {
+  const catalog = getResolvedCatalog(locale, namespaces, getCatalogVersion());
+  const translator = createTranslatorFromMessages(catalog.locale, catalog.messages);
+
+  return {
+    locale: catalog.locale,
+    fallbackChain: catalog.fallbackChain,
+    messages: catalog.messages,
+    t(key, values) {
+      const value = translator.t(key, values);
+      if (value === key && process.env.NODE_ENV !== "production") {
+        console.warn(`[i18n] Missing translation key "${key}" for locale "${catalog.locale}".`);
+      }
+      return value;
+    },
+  };
+}
+
+export function getClientI18nPayload(locale: string, namespaces: readonly I18nNamespace[]) {
+  const catalog = getResolvedCatalog(locale, namespaces, getCatalogVersion());
+  return {
+    locale: catalog.locale,
+    messages: catalog.messages,
+  };
+}