Fix: Fallback to English (not German) for unsupported languages

When a language like PT, FR, IT is selected but has no translation
in exerciseTranslations.ts, the system now falls back to English
instead of German. English is more universally understood.

Applies to: exercise explanations, button labels, instructions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

studio-v2/lib/NativeLanguageContext.tsx

@@ -39,7 +39,7 @@ export function NativeLanguageProvider({ children }: { children: React.ReactNode
   const t = useCallback((key: ExerciseKey): string => {
     const entry = exerciseT[key]
     if (!entry) return key
-    return (entry as Record<string, string>)[nativeLang] || entry.de || key
+    return (entry as Record<string, string>)[nativeLang] || entry.en || entry.de || key
   }, [nativeLang])
 
   const wordInNative = useCallback((translations?: Record<string, any>): string => {