fix(sandbox): wait for locale registration before rendering i18n UI

Avoid a flash of English copy when the sandbox locale changes after mount.

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: sampotts

apps/sandbox/templates/react-video/main.tsx

@@ -34,14 +34,14 @@ function App() {
   const loop = useLoop();
   const preload = usePreload();
   const locale = useLocale();
-  const [ready, setReady] = useState(false);
+  const [registeredLocale, setRegisteredLocale] = useState<string | null>(null);
 
   useEffect(() => {
     ensureSandboxLocale(locale);
-    setReady(true);
+    setRegisteredLocale(locale);
   }, [locale]);
 
-  if (!ready) {
+  if (registeredLocale !== locale) {
     return null;
   }