Add central exercise translation system (7 languages, 30+ keys)

useNativeLanguage.ts: Hook that reads bp_native_language from
localStorage and provides t(key) for translated UI text and
wordInNative() for vocabulary translations.

exerciseTranslations.ts: All exercise UI strings in DE/EN/TR/AR/UK/RU/PL.
Buttons (Richtig/Falsch), instructions, labels, result texts.

Next: Wire into all 9 exercise pages for trilingual display.

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

studio-v2/lib/useNativeLanguage.ts

@@ -0,0 +1,38 @@
+'use client'
+
+import { useState, useEffect } from 'react'
+import { exerciseT, type ExerciseKey } from './exerciseTranslations'
+
+const STORAGE_KEY = 'bp_native_language'
+
+/**
+ * Hook to read the user's native language.
+ * Returns translation helper for exercise UI texts.
+ */
+export function useNativeLanguage() {
+  const [nativeLang, setNativeLang] = useState('de')
+
+  useEffect(() => {
+    const stored = localStorage.getItem(STORAGE_KEY)
+    if (stored) setNativeLang(stored)
+  }, [])
+
+  const isThirdLanguage = nativeLang !== 'de' && nativeLang !== 'en'
+
+  /** Get translated exercise UI text */
+  const t = (key: ExerciseKey): string => {
+    const entry = exerciseT[key]
+    if (!entry) return key
+    return (entry as Record<string, string>)[nativeLang] || entry.de || key
+  }
+
+  /** Get native translation of a vocab word from translations JSONB */
+  const wordInNative = (translations?: Record<string, any>): string => {
+    if (!translations || !isThirdLanguage) return ''
+    const entry = translations[nativeLang]
+    if (!entry) return ''
+    return typeof entry === 'string' ? entry : entry.text || ''
+  }
+
+  return { nativeLang, isThirdLanguage, t, wordInNative }
+}