diff --git a/frontend/src/components/workspace/settings/appearance-settings-page.tsx b/frontend/src/components/workspace/settings/appearance-settings-page.tsx
index 125fdae..2e76d68 100644
--- a/frontend/src/components/workspace/settings/appearance-settings-page.tsx
+++ b/frontend/src/components/workspace/settings/appearance-settings-page.tsx
@@ -12,7 +12,7 @@ import {
   SelectValue,
 } from "@/components/ui/select";
 import { Separator } from "@/components/ui/separator";
-import { enUS, zhCN, type Locale } from "@/core/i18n";
+import { enUS, isLocale, zhCN, type Locale } from "@/core/i18n";
 import { useI18n } from "@/core/i18n/hooks";
 import { cn } from "@/lib/utils";
 
@@ -89,7 +89,11 @@ export function AppearanceSettingsPage() {
       >
         <Select
           value={locale}
-          onValueChange={(value) => changeLocale(value as Locale)}
+          onValueChange={(value) => {
+            if (isLocale(value)) {
+              changeLocale(value);
+            }
+          }}
         >
           <SelectTrigger className="w-[220px]">
             <SelectValue />
diff --git a/frontend/src/core/i18n/hooks.ts b/frontend/src/core/i18n/hooks.ts
index f9e252f..b96acb0 100644
--- a/frontend/src/core/i18n/hooks.ts
+++ b/frontend/src/core/i18n/hooks.ts
@@ -7,7 +7,13 @@ import { getLocaleFromCookie, setLocaleInCookie } from "./cookies";
 import { enUS } from "./locales/en-US";
 import { zhCN } from "./locales/zh-CN";
 
-import { detectLocale, type Locale, type Translations } from "./index";
+import {
+  DEFAULT_LOCALE,
+  detectLocale,
+  normalizeLocale,
+  type Locale,
+  type Translations,
+} from "./index";
 
 const translations: Record<Locale, Translations> = {
   "en-US": enUS,
@@ -17,7 +23,7 @@ const translations: Record<Locale, Translations> = {
 export function useI18n() {
   const { locale, setLocale } = useI18nContext();
 
-  const t = translations[locale];
+  const t = translations[locale] ?? translations[DEFAULT_LOCALE];
 
   const changeLocale = (newLocale: Locale) => {
     setLocale(newLocale);
@@ -26,12 +32,19 @@ export function useI18n() {
 
   // Initialize locale on mount
   useEffect(() => {
-    const saved = getLocaleFromCookie() as Locale | null;
-    if (!saved) {
-      const detected = detectLocale();
-      setLocale(detected);
-      setLocaleInCookie(detected);
+    const saved = getLocaleFromCookie();
+    if (saved) {
+      const normalizedSaved = normalizeLocale(saved);
+      setLocale(normalizedSaved);
+      if (saved !== normalizedSaved) {
+        setLocaleInCookie(normalizedSaved);
+      }
+      return;
     }
+
+    const detected = detectLocale();
+    setLocale(detected);
+    setLocaleInCookie(detected);
   }, [setLocale]);
 
   return {
diff --git a/frontend/src/core/i18n/index.ts b/frontend/src/core/i18n/index.ts
index 90f03ec..c301cee 100644
--- a/frontend/src/core/i18n/index.ts
+++ b/frontend/src/core/i18n/index.ts
@@ -1,23 +1,11 @@
 export { enUS } from "./locales/en-US";
 export { zhCN } from "./locales/zh-CN";
 export type { Translations } from "./locales/types";
-
-export type Locale = "en-US" | "zh-CN";
-
-// Helper function to detect browser locale
-export function detectLocale(): Locale {
-  if (typeof window === "undefined") {
-    return "en-US";
-  }
-
-  const browserLang =
-    navigator.language ||
-    (navigator as unknown as { userLanguage: string }).userLanguage;
-
-  // Check if browser language is Chinese (zh, zh-CN, zh-TW, etc.)
-  if (browserLang.toLowerCase().startsWith("zh")) {
-    return "zh-CN";
-  }
-
-  return "en-US";
-}
+export {
+  DEFAULT_LOCALE,
+  SUPPORTED_LOCALES,
+  detectLocale,
+  isLocale,
+  normalizeLocale,
+} from "./locale";
+export type { Locale } from "./locale";
diff --git a/frontend/src/core/i18n/locale.ts b/frontend/src/core/i18n/locale.ts
new file mode 100644
index 0000000..7fbde14
--- /dev/null
+++ b/frontend/src/core/i18n/locale.ts
@@ -0,0 +1,36 @@
+export const SUPPORTED_LOCALES = ["en-US", "zh-CN"] as const;
+export type Locale = (typeof SUPPORTED_LOCALES)[number];
+export const DEFAULT_LOCALE: Locale = "en-US";
+
+export function isLocale(value: string): value is Locale {
+  return (SUPPORTED_LOCALES as readonly string[]).includes(value);
+}
+
+export function normalizeLocale(locale: string | null | undefined): Locale {
+  if (!locale) {
+    return DEFAULT_LOCALE;
+  }
+
+  if (isLocale(locale)) {
+    return locale;
+  }
+
+  if (locale.toLowerCase().startsWith("zh")) {
+    return "zh-CN";
+  }
+
+  return DEFAULT_LOCALE;
+}
+
+// Helper function to detect browser locale
+export function detectLocale(): Locale {
+  if (typeof window === "undefined") {
+    return DEFAULT_LOCALE;
+  }
+
+  const browserLang =
+    navigator.language ||
+    (navigator as unknown as { userLanguage: string }).userLanguage;
+
+  return normalizeLocale(browserLang);
+}
diff --git a/frontend/src/core/i18n/server.ts b/frontend/src/core/i18n/server.ts
index a03efd7..3600bd7 100644
--- a/frontend/src/core/i18n/server.ts
+++ b/frontend/src/core/i18n/server.ts
@@ -1,9 +1,17 @@
 import { cookies } from "next/headers";
 
-export type Locale = "en-US" | "zh-CN";
+import { normalizeLocale, type Locale } from "./locale";
 
 export async function detectLocaleServer(): Promise<Locale> {
   const cookieStore = await cookies();
-  const locale = cookieStore.get("locale")?.value ?? "en-US";
-  return locale as Locale;
+  let locale = cookieStore.get("locale")?.value;
+  if (locale !== undefined) {
+    try {
+      locale = decodeURIComponent(locale);
+    } catch {
+      // Keep raw cookie value when decoding fails.
+    }
+  }
+
+  return normalizeLocale(locale);
 }