diff --git a/src/app/[locale]/[...rest]/page.tsx b/src/app/[locale]/[...rest]/page.tsx
new file mode 100644
index 0000000..8382a73
--- /dev/null
+++ b/src/app/[locale]/[...rest]/page.tsx
@@ -0,0 +1,9 @@
+import { notFound } from "next/navigation";
+
+// Unmatched URLs inside a matched [locale] segment ("/en/typo") bubble to the
+// root _not-found otherwise — Next only renders a segment's not-found.tsx on
+// a programmatic notFound() call. This catch-all throws it so the translated
+// [locale]/not-found.tsx handles unknown paths (next-intl recommended pattern).
+export default function CatchAllPage() {
+  notFound();
+}
diff --git a/src/app/layout.tsx b/src/app/[locale]/layout.tsx
similarity index 63%
rename from src/app/layout.tsx
rename to src/app/[locale]/layout.tsx
index 15644ae..5b44a26 100644
--- a/src/app/layout.tsx
+++ b/src/app/[locale]/layout.tsx
@@ -1,25 +1,37 @@
 import type { Metadata, Viewport } from "next";
+import { notFound } from "next/navigation";
+import { getTranslations } from "next-intl/server";
+import { hasLocale, NextIntlClientProvider } from "next-intl";
 import { NuqsAdapter } from "nuqs/adapters/next/app";
 import { ThemeProvider, ThemedToaster } from "@/providers/ThemeProvider";
-import { AuthRedirectInstaller } from "./components/AuthRedirectInstaller";
+import { AuthRedirectInstaller } from "../components/AuthRedirectInstaller";
 import { THEME_STORAGE_KEY } from "@/lib/theme";
+import { routing } from "@/i18n/routing";
 import "katex/dist/katex.min.css";
 import "@mescius/spread-sheets/styles/gc.spread.sheets.excel2013white.css";
 import localFont from "next/font/local";
-import "./globals.css";
+import "../globals.css";
 
 export const dynamic = "force-dynamic";
 
-export async function generateMetadata(): Promise<Metadata> {
+export async function generateMetadata({
+  params,
+}: {
+  params: Promise<{ locale: string }>;
+}): Promise<Metadata> {
+  const { locale: rawLocale } = await params;
   const { getSystemConfig, brandingVersion } = await import(
     "@/lib/server/systemConfig"
   );
+  const locale = hasLocale(routing.locales, rawLocale)
+    ? rawLocale
+    : routing.defaultLocale;
+  const t = await getTranslations({ locale, namespace: "common" });
   const config = getSystemConfig();
   const faviconVersion = brandingVersion("favicon");
   return {
     title: `${config.branding.wordmark} WebUI`,
-    description:
-      "Web UI for EvoScientist — a self-evolving AI scientist built on DeepAgents/LangGraph.",
+    description: t("metadataDescription"),
     icons:
       faviconVersion > 0
         ? { icon: `/api/system/branding/asset/favicon?v=${faviconVersion}` }
@@ -38,7 +50,7 @@ export const viewport: Viewport = {
 // Self-hosted Inter variable font; exposes --font-inter, consumed by globals.css.
 // No CJK glyphs — Chinese text falls through to PingFang/YaHei in the stack.
 const inter = localFont({
-  src: "./fonts/InterVariable.woff2",
+  src: "../fonts/InterVariable.woff2",
   variable: "--font-inter",
   display: "swap",
 });
@@ -50,14 +62,20 @@ const themeScript = `(function(){var k=${JSON.stringify(
   THEME_STORAGE_KEY
 )};var t="system";try{t=localStorage.getItem(k)||"system";}catch(_){}var d=t==="dark"||(t!=="light"&&window.matchMedia("(prefers-color-scheme: dark)").matches);var e=document.documentElement;e.classList.toggle("dark",d);e.style.colorScheme=d?"dark":"light";})();`;
 
-export default function RootLayout({
+export default async function RootLayout({
   children,
+  params,
 }: {
   children: React.ReactNode;
+  params: Promise<{ locale: string }>;
 }) {
+  const { locale } = await params;
+  if (!hasLocale(routing.locales, locale)) {
+    notFound();
+  }
   return (
     <html
-      lang="en"
+      lang={locale}
       suppressHydrationWarning
     >
       <body
@@ -65,13 +83,15 @@ export default function RootLayout({
         className={inter.variable}
       >
         <script dangerouslySetInnerHTML={{ __html: themeScript }} />
-        <NuqsAdapter>
-          <ThemeProvider>
-            <AuthRedirectInstaller />
-            {children}
-            <ThemedToaster />
-          </ThemeProvider>
-        </NuqsAdapter>
+        <NextIntlClientProvider locale={locale}>
+          <NuqsAdapter>
+            <ThemeProvider>
+              <AuthRedirectInstaller />
+              {children}
+              <ThemedToaster />
+            </ThemeProvider>
+          </NuqsAdapter>
+        </NextIntlClientProvider>
       </body>
     </html>
   );
diff --git a/src/app/login/page.tsx b/src/app/[locale]/login/page.tsx
similarity index 97%
rename from src/app/login/page.tsx
rename to src/app/[locale]/login/page.tsx
index a6cfa1d..de1693a 100644
--- a/src/app/login/page.tsx
+++ b/src/app/[locale]/login/page.tsx
@@ -4,6 +4,7 @@ import { FormEvent, useState } from "react";
 import { Button } from "@/components/ui/button";
 import { Input } from "@/components/ui/input";
 import { MarkdownContent } from "@/app/components/MarkdownContent";
+import { localizedPath } from "@/lib/localePath";
 import {
   DEFAULT_WORDMARK,
   logoSrc,
@@ -70,7 +71,9 @@ export default function LoginPage() {
         }
         throw new Error(body.error || "Unable to sign in.");
       }
-      window.location.replace(body.redirectTo || "/");
+      window.location.replace(
+        body.redirectTo || localizedPath("/", window.location.pathname)
+      );
     } catch (reason) {
       setError(reason instanceof Error ? reason.message : "Unable to sign in.");
       setSubmitting(false);
diff --git a/src/app/[locale]/not-found.tsx b/src/app/[locale]/not-found.tsx
new file mode 100644
index 0000000..a7ccd06
--- /dev/null
+++ b/src/app/[locale]/not-found.tsx
@@ -0,0 +1,10 @@
+import { useTranslations } from "next-intl";
+
+export default function NotFound() {
+  const t = useTranslations("common");
+  return (
+    <main className="flex min-h-screen items-center justify-center bg-background p-5">
+      <p className="text-sm text-muted-foreground">{t("notFound")}</p>
+    </main>
+  );
+}
diff --git a/src/app/page.tsx b/src/app/[locale]/page.tsx
similarity index 99%
rename from src/app/page.tsx
rename to src/app/[locale]/page.tsx
index ea310e3..64836c2 100644
--- a/src/app/page.tsx
+++ b/src/app/[locale]/page.tsx
@@ -55,6 +55,7 @@ import {
   usePublicSystemConfig,
 } from "@/app/hooks/usePublicSystemConfig";
 import type { MainChatReporter } from "@/lib/asyncAgents";
+import { localizedPath } from "@/lib/localePath";
 import { cn } from "@/lib/utils";
 
 const FALLBACK_ASSISTANT_ID = "EvoScientist";
@@ -217,7 +218,7 @@ function HomePageInner({ authEnabled, isAdmin }: HomePageInnerProps) {
         credentials: "same-origin",
       });
     } finally {
-      window.location.assign("/login");
+      window.location.assign(localizedPath("/login", window.location.pathname));
     }
   }, []);
   const handleDashboardNav = useCallback(
diff --git a/src/lib/authRedirect.test.ts b/src/lib/authRedirect.test.ts
index 7b81f18..4e0b575 100644
--- a/src/lib/authRedirect.test.ts
+++ b/src/lib/authRedirect.test.ts
@@ -32,6 +32,14 @@ describe("shouldRedirectFor", () => {
     expect(shouldRedirectFor("/api/auth/me", 401, "/login")).toBe(false);
   });
 
+  it("does not redirect when already on a localized login page", () => {
+    expect(shouldRedirectFor("/api/threads", 401, "/zh/login")).toBe(false);
+  });
+
+  it("still redirects for API 401s from a localized page", () => {
+    expect(shouldRedirectFor("/api/threads", 401, "/zh")).toBe(true);
+  });
+
   it("ignores unparseable urls", () => {
     expect(shouldRedirectFor("http://[", 401, "/")).toBe(false);
   });
diff --git a/src/lib/authRedirect.ts b/src/lib/authRedirect.ts
index b1080bc..72c7eb8 100644
--- a/src/lib/authRedirect.ts
+++ b/src/lib/authRedirect.ts
@@ -1,10 +1,12 @@
+import { localizedPath, stripLocalePrefix } from "@/lib/localePath";
+
 export function shouldRedirectFor(
   url: string,
   status: number,
   pathname: string
 ): boolean {
   if (status !== 401) return false;
-  if (pathname.startsWith("/login")) return false;
+  if (stripLocalePrefix(pathname).startsWith("/login")) return false;
   let path: string;
   try {
     path = new URL(url, "http://localhost").pathname;
@@ -41,7 +43,9 @@ export function installAuthRedirect(): () => void {
         const next = encodeURIComponent(
           window.location.pathname + window.location.search
         );
-        window.location.replace(`/login?next=${next}`);
+        window.location.replace(
+          `${localizedPath("/login", window.location.pathname)}?next=${next}`
+        );
       }
     } catch {
       // Redirect failures must not break the caller's response handling.
diff --git a/src/lib/localePath.test.ts b/src/lib/localePath.test.ts
new file mode 100644
index 0000000..fba7f64
--- /dev/null
+++ b/src/lib/localePath.test.ts
@@ -0,0 +1,56 @@
+import { describe, expect, it } from "vitest";
+import {
+  hasFileExtension,
+  localePrefixOf,
+  localizedPath,
+  stripLocalePrefix,
+} from "./localePath";
+
+describe("stripLocalePrefix", () => {
+  it.each([
+    ["/zh/login", "/login"],
+    ["/en", "/"],
+    ["/zh", "/"],
+    ["/zh/", "/"],
+    ["/en/chat", "/chat"],
+    ["/login", "/login"],
+    ["/", "/"],
+    ["/fr/login", "/fr/login"],
+    ["/zho", "/zho"],
+    ["/api/system/config", "/api/system/config"],
+  ])("%s → %s", (input, expected) => {
+    expect(stripLocalePrefix(input)).toBe(expected);
+  });
+});
+
+describe("localePrefixOf", () => {
+  it.each([
+    ["/zh/login", "/zh"],
+    ["/zh", "/zh"],
+    ["/en/chat", "/en"],
+    ["/login", "/en"],
+    ["/", "/en"],
+  ])("%s → %s", (input, expected) => {
+    expect(localePrefixOf(input)).toBe(expected);
+  });
+});
+
+describe("localizedPath", () => {
+  it("prefixes a locale-less path with the current locale", () => {
+    expect(localizedPath("/login", "/zh/chat")).toBe("/zh/login");
+    expect(localizedPath("/", "/zh/chat")).toBe("/zh");
+    expect(localizedPath("/login", "/chat")).toBe("/en/login");
+  });
+});
+
+describe("hasFileExtension", () => {
+  it.each([
+    ["/icon.png", true],
+    ["/evoscientist-logo.png", true],
+    ["/zh/chat", false],
+    ["/zh", false],
+    ["/api/workspace/render/x.html", true],
+  ])("%s → %s", (input, expected) => {
+    expect(hasFileExtension(input)).toBe(expected);
+  });
+});
diff --git a/src/lib/localePath.ts b/src/lib/localePath.ts
new file mode 100644
index 0000000..bee8a63
--- /dev/null
+++ b/src/lib/localePath.ts
@@ -0,0 +1,25 @@
+import { routing } from "@/i18n/routing";
+
+const LOCALES_RE = new RegExp(`^/(${routing.locales.join("|")})(?=/|$)`);
+
+/** "/zh/login" → "/login"; paths without an en/zh prefix are returned as-is. */
+export function stripLocalePrefix(pathname: string): string {
+  const match = LOCALES_RE.exec(pathname);
+  return match ? pathname.slice(match[0].length) || "/" : pathname;
+}
+
+/** "/zh/x" → "/zh"; unprefixed paths yield the default locale's prefix. */
+export function localePrefixOf(pathname: string): string {
+  const match = LOCALES_RE.exec(pathname);
+  return match ? `/${match[1]}` : `/${routing.defaultLocale}`;
+}
+
+/** Prefix a locale-less app path with the locale found in currentPathname. */
+export function localizedPath(path: string, currentPathname: string): string {
+  return `${localePrefixOf(currentPathname)}${path === "/" ? "" : path}`;
+}
+
+/** Static-asset lookalikes ("/icon.png") bypass the intl middleware. */
+export function hasFileExtension(pathname: string): boolean {
+  return /\.[^/]+$/.test(pathname);
+}
diff --git a/src/proxy.ts b/src/proxy.ts
index 04bd592..69205f9 100644
--- a/src/proxy.ts
+++ b/src/proxy.ts
@@ -1,10 +1,13 @@
 import { type NextRequest, NextResponse } from "next/server";
+import createMiddleware from "next-intl/middleware";
 import {
   AUTH_COOKIE_NAME,
   AUTH_LOGIN_PATH,
   authSecret,
 } from "@/lib/auth";
 import { effectiveSecurity } from "@/lib/server/systemConfig";
+import { routing } from "@/i18n/routing";
+import { hasFileExtension, stripLocalePrefix } from "@/lib/localePath";
 
 interface SessionPayload {
   username: string;
@@ -102,24 +105,48 @@ function configurationError(request: NextRequest) {
   });
 }
 
+const intlMiddleware = createMiddleware(routing);
+
+// Page paths get locale routing; API routes and static-asset lookalikes keep
+// the pre-i18n behavior. Auth decisions run against the locale-stripped path
+// so /zh/login is public exactly like /login, and the login redirect carries
+// the visitor's current locale prefix.
 export async function proxy(request: NextRequest) {
-  if (!effectiveSecurity().authEnabled || isPublicPath(request.nextUrl.pathname)) {
-    return NextResponse.next();
+  const { pathname } = request.nextUrl;
+  const isApi = pathname.startsWith("/api/");
+  const authPath = stripLocalePrefix(pathname);
+  const passThrough = () =>
+    isApi || hasFileExtension(pathname)
+      ? NextResponse.next()
+      : intlMiddleware(request);
+
+  if (!effectiveSecurity().authEnabled || isPublicPath(authPath)) {
+    return passThrough();
   }
   if (!authSecret() || authSecret()!.length < 32) {
     return configurationError(request);
   }
   const token = request.cookies.get(AUTH_COOKIE_NAME)?.value;
-  if (await isValidSession(token)) return NextResponse.next();
+  if (await isValidSession(token)) return passThrough();
 
-  if (request.nextUrl.pathname.startsWith("/api/")) {
+  if (isApi) {
     return NextResponse.json(
       { error: "Authentication required." },
       { status: 401 }
     );
   }
-  const loginUrl = new URL(AUTH_LOGIN_PATH, request.url);
-  const returnTo = `${request.nextUrl.pathname}${request.nextUrl.search}`;
+  // Prefer the URL's locale; else the remembered NEXT_LOCALE cookie; else the
+  // default. Accept-Language negotiation happens once the browser lands on
+  // the login page (public → intl middleware handles it).
+  const cookieLocale = request.cookies.get("NEXT_LOCALE")?.value;
+  const prefix =
+    authPath !== pathname
+      ? `/${pathname.split("/")[1]}`
+      : cookieLocale === "en" || cookieLocale === "zh"
+        ? `/${cookieLocale}`
+        : `/${routing.defaultLocale}`;
+  const loginUrl = new URL(`${prefix}${AUTH_LOGIN_PATH}`, request.url);
+  const returnTo = `${pathname}${request.nextUrl.search}`;
   loginUrl.searchParams.set("next", returnTo);
   return NextResponse.redirect(loginUrl);
 }