feature: localization's first implmentation

frontend/src/components/i18n/LocaleSync.tsx

@@ -0,0 +1,27 @@
+'use client';
+
+import { useEffect } from 'react';
+import { useLocale } from 'next-intl';
+import { useAuth } from '@/lib/hooks/useAuth';
+import { usePathname, useRouter } from '@/i18n/navigation';
+import type { AppLocale } from '@/i18n/routing';
+import { isAppLocale } from '@/i18n/routing';
+
+/** Redirect authenticated users to their saved profile language when it differs from the URL. */
+export function LocaleSync() {
+  const locale = useLocale();
+  const router = useRouter();
+  const pathname = usePathname();
+  const { user, isAuthReady } = useAuth();
+
+  useEffect(() => {
+    if (!isAuthReady || !user?.language) return;
+
+    const preferred = user.language;
+    if (!isAppLocale(preferred) || preferred === locale) return;
+
+    router.replace(pathname, { locale: preferred as AppLocale });
+  }, [isAuthReady, user?.language, locale, pathname, router]);
+
+  return null;
+}