diff --git a/app/api/create-client-user/route.ts b/app/api/create-client-user/route.ts
index e54eb11..d2e6fa8 100644
--- a/app/api/create-client-user/route.ts
+++ b/app/api/create-client-user/route.ts
@@ -17,8 +17,8 @@ export async function POST(request: Request) {
   }
 
   try {
-    const { email, client_id: clientId } = await request.json();
-    const invitation = await createPortalInvitation(actor, { email, clientId });
+    const { email, client_id: clientId, locale } = await request.json();
+    const invitation = await createPortalInvitation(actor, { email, clientId, locale });
 
     return NextResponse.json({ success: true, invitation }, { status: 201 });
   } catch (error) {
diff --git a/app/api/portal-clients/[clientId]/locale/route.ts b/app/api/portal-clients/[clientId]/locale/route.ts
new file mode 100644
index 0000000..7f550af
--- /dev/null
+++ b/app/api/portal-clients/[clientId]/locale/route.ts
@@ -0,0 +1,31 @@
+import { NextResponse } from "next/server";
+import {
+  PortalInvitationError,
+  setClientPortalLocale,
+} from "@/server/auth/invitations";
+import { getSessionContextFromHeaders } from "@/server/auth/session";
+
+export async function PATCH(request: Request, { params }: { params: Promise<{ clientId: string }> }) {
+  const actor = await getSessionContextFromHeaders(new Headers(request.headers));
+
+  if (!actor) {
+    return NextResponse.json({ error: "Oturum gerekli." }, { status: 401 });
+  }
+
+  try {
+    const body = await request.json();
+    const result = setClientPortalLocale(actor, (await params).clientId, body.locale);
+    return NextResponse.json(result);
+  } catch (error) {
+    if (error instanceof SyntaxError) {
+      return NextResponse.json({ error: "Geçersiz JSON gövdesi." }, { status: 400 });
+    }
+    if (error instanceof PortalInvitationError) {
+      const status = error.code === "FORBIDDEN" ? 403 : error.code === "CLIENT_NOT_FOUND" ? 404 : 400;
+      return NextResponse.json({ error: error.message, code: error.code }, { status });
+    }
+
+    console.error("Client portal locale update failed", error);
+    return NextResponse.json({ error: "Portal dili güncellenemedi." }, { status: 500 });
+  }
+}
diff --git a/app/api/portal-invitations/route.ts b/app/api/portal-invitations/route.ts
index 2c825cd..8838f82 100644
--- a/app/api/portal-invitations/route.ts
+++ b/app/api/portal-invitations/route.ts
@@ -17,6 +17,7 @@ export async function POST(request: Request) {
     const invitation = await createPortalInvitation(actor, {
       clientId: body.clientId,
       email: body.email,
+      locale: body.locale,
       expiresInHours: body.expiresInHours,
     });
 
diff --git a/app/invite/[token]/actions.ts b/app/invite/[token]/actions.ts
index 1ced07b..6c0d0f3 100644
--- a/app/invite/[token]/actions.ts
+++ b/app/invite/[token]/actions.ts
@@ -1,10 +1,20 @@
 "use server";
 
+import { cookies } from "next/headers";
 import { redirect } from "next/navigation";
 import {
   acceptPortalInvitation,
+  getPortalInvitationPreview,
   PortalInvitationError,
 } from "@/server/auth/invitations";
+import { buildLocaleCookie } from "@/server/i18n/locale";
+
+function inviteErrorCode(error: unknown): string {
+  if (!(error instanceof PortalInvitationError)) return "auth.messages.portalInviteFailed";
+  if (error.code === "INVITATION_EXPIRED") return "auth.invite.expired";
+  if (error.code === "INVITATION_NOT_PENDING") return "auth.invite.accepted";
+  return "auth.messages.portalInviteFailed";
+}
 
 export async function acceptInvitation(formData: FormData) {
   const token = String(formData.get("token") ?? "");
@@ -14,14 +24,10 @@ export async function acceptInvitation(formData: FormData) {
   try {
     await acceptPortalInvitation({ token, displayName, password });
   } catch (error) {
-    const message =
-      error instanceof PortalInvitationError
-        ? error.message
-        : "Portal hesabı oluşturulamadı.";
-    redirect(`/invite/${encodeURIComponent(token)}?error=true&message=${encodeURIComponent(message)}`);
+    redirect(`/invite/${encodeURIComponent(token)}?error=true&code=${inviteErrorCode(error)}`);
   }
 
-  redirect(
-    `/login?message=${encodeURIComponent("Portal hesabın oluşturuldu. Şimdi giriş yapabilirsin.")}`,
-  );
+  const locale = getPortalInvitationPreview(token)?.locale ?? "tr";
+  (await cookies()).set(buildLocaleCookie(locale));
+  redirect("/login?code=auth.invite.success");
 }
diff --git a/app/invite/[token]/page.tsx b/app/invite/[token]/page.tsx
index e93a3c5..ea90175 100644
--- a/app/invite/[token]/page.tsx
+++ b/app/invite/[token]/page.tsx
@@ -9,6 +9,7 @@ import { Input, Label } from "poyraz-ui/atoms";
 import { Alert, AlertDescription } from "poyraz-ui/molecules";
 import { getPortalInvitationPreview } from "@/server/auth/invitations";
 import { getPublicBranding } from "@/server/branding/runtime";
+import { createTranslator } from "@/server/i18n/translator";
 
 export const dynamic = "force-dynamic";
 
@@ -17,7 +18,7 @@ export default async function InvitationPage({
   searchParams,
 }: {
   params: Promise<{ token: string }>;
-  searchParams: Promise<{ error?: string; message?: string }>;
+  searchParams: Promise<{ error?: string; code?: string; message?: string }>;
 }) {
   const { token } = await params;
   const invitation = getPortalInvitationPreview(token);
@@ -27,28 +28,47 @@ export default async function InvitationPage({
     notFound();
   }
 
+  const t = createTranslator(invitation.locale, ["auth"]).t;
   const query = await searchParams;
+  const queryCode = query.code ?? null;
+  const queryMessage = queryCode ? t(queryCode) : query.message;
+  const resolvedQueryMessage = queryMessage === queryCode ? query.message : queryMessage;
+  const marketing = {
+    headline: t("auth.marketing.headline"),
+    description: t("auth.marketing.description", { app: branding.organizationName ?? branding.applicationName }),
+    openSource: t("auth.marketing.openSource"),
+    github: t("auth.marketing.github"),
+    via: t("auth.marketing.via"),
+    builtBy: t("auth.marketing.builtBy"),
+    highlights: [
+      t("auth.highlights.clients"),
+      t("auth.highlights.calendar"),
+      t("auth.highlights.finance"),
+      t("auth.highlights.reports"),
+    ] as [string, string, string, string],
+  };
   const isUsable = invitation.status === "pending";
   const unavailableMessage =
     invitation.status === "expired"
-      ? "Bu davetin süresi dolmuş. Freelancer'dan yeni bir bağlantı istemelisin."
+      ? t("auth.invite.expired")
       : invitation.status === "accepted"
-        ? "Bu davet daha önce kullanılmış. Hesabınla giriş yapabilirsin."
+        ? t("auth.invite.accepted")
         : invitation.status === "revoked"
-          ? "Bu davet iptal edilmiş. Freelancer'dan yeni bir bağlantı istemelisin."
+          ? t("auth.invite.revoked")
           : null;
 
   return (
     <>
-      {query.error && query.message ? <ErrorToaster message={query.message} /> : null}
+      {query.error && resolvedQueryMessage ? <ErrorToaster message={resolvedQueryMessage} /> : null}
       <AuthPageShell
         branding={{
           applicationName: branding.organizationName ?? branding.applicationName,
           lightLogoUrl: branding.lightLogoUrl,
           darkLogoUrl: branding.darkLogoUrl,
         }}
-        title="Müşteri portalına katıl"
-        description="Davet edilen hesabın için adını ve şifreni belirle."
+        title={t("auth.invite.title")}
+        description={t("auth.invite.description")}
+        marketing={marketing}
         form={
           isUsable ? (
             <form className="space-y-6">
@@ -57,28 +77,28 @@ export default async function InvitationPage({
                 <div className="space-y-2">
                   <Label htmlFor="email" className="flex items-center gap-2">
                     <Mail className="h-4 w-4 text-muted-foreground" />
-                    E-posta
+                    {t("auth.invite.email")}
                   </Label>
                   <Input id="email" type="email" value={invitation.email} disabled />
                 </div>
                 <div className="space-y-2">
                   <Label htmlFor="displayName" className="flex items-center gap-2">
                     <UserRound className="h-4 w-4 text-muted-foreground" />
-                    Ad soyad
+                    {t("auth.invite.displayName")}
                   </Label>
                   <Input id="displayName" name="displayName" required maxLength={120} />
                 </div>
                 <div className="space-y-2">
                   <Label htmlFor="password" className="flex items-center gap-2">
                     <LockKeyhole className="h-4 w-4 text-muted-foreground" />
-                    Şifre
+                    {t("auth.invite.password")}
                   </Label>
                   <Input id="password" name="password" type="password" required minLength={8} maxLength={128} />
-                  <p className="text-xs text-muted-foreground">En az 8 karakter kullan.</p>
+                  <p className="text-xs text-muted-foreground">{t("auth.invite.passwordHelp")}</p>
                 </div>
               </div>
-              <SubmitButton size="lg" formAction={acceptInvitation} className="w-full" pendingText="Hesap oluşturuluyor...">
-                Portal hesabını oluştur
+              <SubmitButton size="lg" formAction={acceptInvitation} className="w-full" pendingText={t("auth.invite.pending")}>
+                {t("auth.invite.submit")}
               </SubmitButton>
             </form>
           ) : (
@@ -90,7 +110,7 @@ export default async function InvitationPage({
         secondaryAction={null}
         footer={
           <Link href="/login" className="text-sm font-medium text-primary hover:text-primary-hover">
-            Giriş sayfasına dön
+            {t("auth.invite.backToLogin")}
           </Link>
         }
       />
diff --git a/app/portal/layout.tsx b/app/portal/layout.tsx
index 6e0567b..e97d801 100644
--- a/app/portal/layout.tsx
+++ b/app/portal/layout.tsx
@@ -1,5 +1,7 @@
 import { PortalShell } from "@/components/layout/portal-shell";
 import { getPublicBranding } from "@/server/branding/runtime";
+import { resolveRequestLocale } from "@/server/i18n/resolver";
+import { createTranslator, getClientI18nPayload } from "@/server/i18n/translator";
 import { getUserPreferences } from "@/server/settings/preferences";
 import { requirePortalBackend } from "@/server/web/portal";
 
@@ -9,6 +11,8 @@ export default async function PortalLayout({
   children: React.ReactNode;
 }>) {
   const { context, actor, service } = await requirePortalBackend();
+  const resolvedLocale = await resolveRequestLocale();
+  const t = createTranslator(resolvedLocale.locale, ["navigation", "portal", "common"]).t;
   const { user, profile } = context;
   const branding = getPublicBranding();
   const preferences = getUserPreferences(actor);
@@ -43,6 +47,22 @@ export default async function PortalLayout({
         avatarUrl: user.image || null,
       }}
       progress={progress}
+      i18n={getClientI18nPayload(resolvedLocale.locale, ["navigation", "portal", "common", "status", "validation"])}
+      labels={{
+        skipToContent: t("navigation.shell.skipToContent"),
+        homeAriaLabel: t("navigation.shell.homeAriaLabel", { app: branding.organizationName ?? branding.applicationName }),
+        mobileMenuAriaLabel: t("navigation.shell.mobileMenuAriaLabel"),
+        mobileMenuTooltip: t("navigation.shell.mobileMenuTooltip"),
+        logoAlt: t("navigation.shell.logoAlt", { app: branding.organizationName ?? branding.applicationName }),
+        progressTitle: t("navigation.shell.progressTitle"),
+        progressValue: t("navigation.shell.progressValue", { progress }),
+        progressAriaLabel: t("navigation.shell.progressAriaLabel"),
+        accountMenuAriaLabel: t("navigation.shell.accountMenuAriaLabel", { name: displayName }),
+        signOut: t("navigation.account.signOut"),
+        signingOut: t("navigation.account.signingOut"),
+        signOutError: t("navigation.account.signOutError"),
+        settings: t("navigation.items.settings"),
+      }}
     >
       {children}
     </PortalShell>
diff --git a/components/layout/portal-shell.tsx b/components/layout/portal-shell.tsx
index b73a1fc..f3e84e8 100644
--- a/components/layout/portal-shell.tsx
+++ b/components/layout/portal-shell.tsx
@@ -1,8 +1,11 @@
 "use client";
 
+import type { ComponentProps } from "react";
 import { AppShell, type AppShellBranding } from "@/components/layout/app-shell";
-import { portalSidebarData } from "@/config/portal-sidebar";
+import { I18nProvider } from "@/components/i18n/i18n-provider";
+import { localizePortalSidebarData } from "@/config/portal-sidebar";
 import type { ColorMode } from "@/lib/color-mode";
+import { createTranslatorFromMessages } from "@/lib/i18n";
 
 type PortalShellProps = {
   branding: AppShellBranding;
@@ -15,20 +18,31 @@ type PortalShellProps = {
     avatarUrl: string | null;
   };
   progress?: number;
+  i18n: {
+    locale: string;
+    messages: Record<string, string>;
+  };
+  labels?: ComponentProps<typeof AppShell>["labels"];
 };
 
-export function PortalShell({ branding, children, colorMode, user, progress }: PortalShellProps) {
+export function PortalShell({ branding, children, colorMode, user, progress, i18n, labels }: PortalShellProps) {
+  const translator = createTranslatorFromMessages(i18n.locale, i18n.messages);
+  const navGroups = localizePortalSidebarData(translator.t);
+
   return (
-    <AppShell
-      branding={branding}
-      colorMode={colorMode}
-      homeHref="/portal"
-      navGroups={portalSidebarData}
-      settingsHref="/portal/settings"
-      user={user}
-      progress={progress}
-    >
-      {children}
-    </AppShell>
+    <I18nProvider locale={i18n.locale} messages={i18n.messages}>
+      <AppShell
+        branding={branding}
+        colorMode={colorMode}
+        homeHref="/portal"
+        navGroups={navGroups}
+        settingsHref="/portal/settings"
+        user={user}
+        progress={progress}
+        labels={labels}
+      >
+        {children}
+      </AppShell>
+    </I18nProvider>
   );
 }
diff --git a/config/portal-sidebar.ts b/config/portal-sidebar.ts
index 5ac71b0..66e6ec7 100644
--- a/config/portal-sidebar.ts
+++ b/config/portal-sidebar.ts
@@ -7,6 +7,7 @@ import {
 
 export type PortalSidebarNavItem = {
   title: string;
+  titleKey?: string;
   href?: string;
   icon?: LucideIcon;
   items?: PortalSidebarNavItem[];
@@ -14,22 +15,36 @@ export type PortalSidebarNavItem = {
 
 export type PortalSidebarNavGroup = {
   title: string;
+  titleKey?: string;
   items: PortalSidebarNavItem[];
 };
 
 export const portalSidebarData: PortalSidebarNavGroup[] = [
   {
     title: "GENEL BAKIŞ",
+    titleKey: "navigation.groups.overview",
     items: [
-      { title: "Dashboard", href: "/portal", icon: Sparkles },
+      { title: "Dashboard", titleKey: "navigation.items.dashboard", href: "/portal", icon: Sparkles },
     ],
   },
   {
     title: "SÜREÇLER",
+    titleKey: "navigation.groups.processes",
     items: [
-      { title: "Projeleriniz", href: "/portal/projects", icon: FolderKanban },
-      { title: "Yapılan Görevler", href: "/portal/tasks", icon: CheckSquare2 },
-      { title: "Revizyon Talepleri", href: "/portal/revisions", icon: Sparkles },
+      { title: "Projeleriniz", titleKey: "navigation.items.portalProjects", href: "/portal/projects", icon: FolderKanban },
+      { title: "Yapılan Görevler", titleKey: "navigation.items.portalTasks", href: "/portal/tasks", icon: CheckSquare2 },
+      { title: "Revizyon Talepleri", titleKey: "navigation.items.portalRevisions", href: "/portal/revisions", icon: Sparkles },
     ],
   }
 ];
+
+export function localizePortalSidebarData(t: (key: string) => string): PortalSidebarNavGroup[] {
+  return portalSidebarData.map((group) => ({
+    ...group,
+    title: group.titleKey ? t(group.titleKey) : group.title,
+    items: group.items.map((item) => ({
+      ...item,
+      title: item.titleKey ? t(item.titleKey) : item.title,
+    })),
+  }));
+}
diff --git a/server/auth/invitations.ts b/server/auth/invitations.ts
index be569a9..5ad8b57 100644
--- a/server/auth/invitations.ts
+++ b/server/auth/invitations.ts
@@ -12,9 +12,11 @@ import {
   appProfiles,
   authAuditEvents,
   clients,
+  instanceLocales,
   portalInvitations,
   session,
   user,
+  userPreferences,
 } from "@/server/db/schema";
 import { getDefaultDisplayName, normalizeAuthEmail } from "@/server/auth/validation";
 
@@ -23,6 +25,7 @@ const DEFAULT_INVITATION_TTL_HOURS = 72;
 const createInvitationSchema = z.object({
   clientId: z.string().trim().min(1).max(128),
   email: z.email().transform(normalizeAuthEmail),
+  locale: z.string().trim().min(2).max(12).default("tr"),
   expiresInHours: z.number().int().min(1).max(168).default(DEFAULT_INVITATION_TTL_HOURS),
 });
 
@@ -56,12 +59,13 @@ export type PortalInvitationPreview = {
   email: string;
   expiresAt: Date;
   status: "pending" | "accepted" | "revoked" | "expired";
+  locale: string;
 };
 
 export async function createPortalInvitation(
   actor: SessionContext,
   input: z.input<typeof createInvitationSchema>,
-): Promise<{ id: number; invitationUrl: string; expiresAt: Date }> {
+): Promise<{ id: number; invitationUrl: string; expiresAt: Date; locale: string }> {
   assertFreelancerActor(actor);
 
   const parsed = parseOrThrow(createInvitationSchema, input);
@@ -83,6 +87,7 @@ export async function createPortalInvitation(
     if (!client) {
       throw new PortalInvitationError("CLIENT_NOT_FOUND", "Müşteri bulunamadı.");
     }
+    const locale = assertPortalReadyLocale(tx, parsed.locale);
 
     if (client.authUserId) {
       throw new PortalInvitationError(
@@ -143,6 +148,7 @@ export async function createPortalInvitation(
         tokenHash,
         clientId: parsed.clientId,
         email: parsed.email,
+        locale,
         status: "pending",
         expiresAt,
         createdByUserId: actor.user.id,
@@ -159,6 +165,7 @@ export async function createPortalInvitation(
         metadata: {
           invitationId: inserted.id,
           clientId: parsed.clientId,
+          locale,
           expiresAt: expiresAt.toISOString(),
         },
       })
@@ -171,6 +178,7 @@ export async function createPortalInvitation(
     id: invitationId,
     invitationUrl: `${getServerConfig().appUrl}/invite/${rawToken}`,
     expiresAt,
+    locale: parsed.locale,
   };
 }
 
@@ -186,6 +194,7 @@ export function getPortalInvitationPreview(rawToken: string): PortalInvitationPr
       email: portalInvitations.email,
       status: portalInvitations.status,
       expiresAt: portalInvitations.expiresAt,
+      locale: portalInvitations.locale,
     })
     .from(portalInvitations)
     .where(eq(portalInvitations.tokenHash, hashInvitationToken(rawToken)))
@@ -214,7 +223,7 @@ export function getPortalInvitationPreview(rawToken: string): PortalInvitationPr
           .values({
             type: "invitation_expired",
             email: invitation.email,
-            metadata: { invitationId: invitation.id },
+          metadata: { invitationId: invitation.id },
           })
           .run();
       }
@@ -356,7 +365,7 @@ export async function acceptPortalInvitation(input: {
 
       const linkedClient = tx
         .update(clients)
-        .set({ authUserId, updatedAt: now })
+        .set({ authUserId, portalLocale: invitation.locale, updatedAt: now })
         .where(
           and(
             eq(clients.id, invitation.clientId),
@@ -372,6 +381,20 @@ export async function acceptPortalInvitation(input: {
         );
       }
 
+      tx.insert(userPreferences)
+        .values({
+          ownerUserId: authUserId,
+          language: invitation.locale,
+        })
+        .onConflictDoUpdate({
+          target: userPreferences.ownerUserId,
+          set: {
+            language: invitation.locale,
+            updatedAt: now.toISOString(),
+          },
+        })
+        .run();
+
       const accepted = tx
         .update(portalInvitations)
         .set({ status: "accepted", acceptedAt: now })
@@ -395,7 +418,7 @@ export async function acceptPortalInvitation(input: {
           type: "invitation_accepted",
           authUserId,
           email: invitation.email,
-          metadata: { invitationId: invitation.id, clientId: invitation.clientId },
+          metadata: { invitationId: invitation.id, clientId: invitation.clientId, locale: invitation.locale },
         })
         .run();
 
@@ -497,6 +520,60 @@ export function setClientPortalAccess(
   });
 }
 
+export function setClientPortalLocale(
+  actor: SessionContext,
+  clientId: string,
+  localeInput: string,
+): { locale: string } {
+  assertFreelancerActor(actor);
+  const { db } = getSqliteConnection();
+
+  return db.transaction((tx) => {
+    const ownedClient = tx
+      .select({ id: clients.id, authUserId: clients.authUserId })
+      .from(clients)
+      .where(and(eq(clients.id, clientId), eq(clients.ownerUserId, actor.user.id)))
+      .get();
+
+    if (!ownedClient) {
+      throw new PortalInvitationError("CLIENT_NOT_FOUND", "Müşteri bulunamadı.");
+    }
+
+    const locale = assertPortalReadyLocale(tx, localeInput);
+    tx.update(clients)
+      .set({ portalLocale: locale, updatedAt: new Date() })
+      .where(eq(clients.id, clientId))
+      .run();
+
+    if (ownedClient.authUserId) {
+      tx.insert(userPreferences)
+        .values({
+          ownerUserId: ownedClient.authUserId,
+          language: locale,
+        })
+        .onConflictDoUpdate({
+          target: userPreferences.ownerUserId,
+          set: {
+            language: locale,
+            updatedAt: new Date().toISOString(),
+          },
+        })
+        .run();
+      tx.delete(session).where(eq(session.userId, ownedClient.authUserId)).run();
+    }
+
+    tx.insert(authAuditEvents)
+      .values({
+        type: "client_locale_updated",
+        authUserId: actor.user.id,
+        metadata: { clientId, locale, targetAuthUserId: ownedClient.authUserId },
+      })
+      .run();
+
+    return { locale };
+  });
+}
+
 export function hashInvitationToken(rawToken: string): string {
   return createHash("sha256").update(rawToken, "utf8").digest("hex");
 }
@@ -534,3 +611,21 @@ async function recordInvitationFailure(email: string | null, reason: string): Pr
     })
     .run();
 }
+
+function assertPortalReadyLocale(
+  tx: Pick<ReturnType<typeof getSqliteConnection>["db"], "select">,
+  localeInput: string,
+): string {
+  const locale = localeInput.trim();
+  const row = tx
+    .select({ code: instanceLocales.code, status: instanceLocales.status })
+    .from(instanceLocales)
+    .where(eq(instanceLocales.code, locale))
+    .get();
+
+  if (!row || row.status !== "active") {
+    throw new PortalInvitationError("INVALID_INPUT", "Portal dili aktif değil.");
+  }
+
+  return row.code;
+}
diff --git a/server/auth/types.ts b/server/auth/types.ts
index 8274201..b1442e9 100644
--- a/server/auth/types.ts
+++ b/server/auth/types.ts
@@ -22,4 +22,5 @@ export type AuthAuditEventType =
   | "invitation_accepted"
   | "invitation_accept_failed"
   | "client_access_disabled"
-  | "client_access_enabled";
+  | "client_access_enabled"
+  | "client_locale_updated";