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/components/learn/ExerciseLayout.tsx

@@ -68,6 +68,7 @@ export function ExerciseLayout({
   const typeKey = exerciseType || title.toLowerCase()
   const explanation = exerciseExplanation
     || explanations[typeKey]?.[nativeLang]
+    || explanations[typeKey]?.['en']
     || explanations[typeKey]?.['de']
     || ''