refactor(vocabService, VocabLessonView): unify answer language classification for multiple-choice questions

- Renamed and refactored the method for classifying answer languages in multiple-choice questions from _classifyMcQuestionSide to _resolveMcAnswerSide for clarity and consistency.
- Updated the logic to classify answer languages based on questionData.answerLanguage and answerLanguageId, improving accuracy in determining the correct language context.
- Adjusted VocabLessonView to utilize the new classification method, ensuring alignment with backend changes and enhancing the handling of distractor options based on answer language.
- Enhanced documentation to clarify the expected input and output for the new method, improving maintainability and understanding of the codebase.

frontend/src/views/social/VocabLessonView.vue

@@ -2166,11 +2166,22 @@ export default {
       }
       return a;
     },
-    /** Muss zu backend/services/vocabService _classifyMcQuestionSide passen */
-    _classifyMcQuestionSide(question) {
-      const q = String(question || '');
-      if (/Wie sagt man\s/i.test(q) || /Übersetze/i.test(q)) return 'target';
-      if (/Was bedeutet/i.test(q)) return 'native';
+    /**
+     * Welche Sprache haben die richtigen MC-Antworten? (Backend: vocabService._resolveMcAnswerSide)
+     * Nur über questionData.answerLanguage ('target'|'native') oder answerLanguageId (1|2).
+     * Ohne diese Felder: 'unknown' — dann kein Distraktor-Randomize (bis Inhalte annotiert sind).
+     */
+    _resolveMcAnswerSide(questionData) {
+      const qd = questionData || {};
+      const raw = qd.answerLanguage;
+      if (typeof raw === 'string') {
+        const s = raw.trim().toLowerCase();
+        if (s === 'target' || s === 'learning' || s === 'l2') return 'target';
+        if (s === 'native' || s === 'l1') return 'native';
+      }
+      const id = qd.answerLanguageId;
+      if (id === 1 || id === '1') return 'target';
+      if (id === 2 || id === '2') return 'native';
       return 'unknown';
     },
     /**
@@ -2181,8 +2192,7 @@ export default {
       const q = this.getQuestionData(exercise);
       const a = this.getAnswerData(exercise);
       if (!q || !a || q.type !== 'multiple_choice' || q.randomizeDistractors === false) return null;
-      const qtext = q.question || '';
-      const side = this._classifyMcQuestionSide(qtext);
+      const side = this._resolveMcAnswerSide(q);
       if (side === 'unknown') return null;
       const options = q.options || [];
       let correctIndices = [];
@@ -2697,12 +2707,30 @@ export default {
 
       const answerField = direction === 'L2R' ? 'reference' : 'learning';
 
+      const allNativeNorm = new Set();
+      const allTargetNorm = new Set();
+      allVocabs.forEach((v) => {
+        if (v.learning) allNativeNorm.add(this.normalizeVocab(v.learning));
+        if (v.reference) allTargetNorm.add(this.normalizeVocab(v.reference));
+      });
+      /** Verhindert gemischte Sprachen in den Optionen (z. B. deutsche Wörter unter Zielsprachen-Distraktoren). */
+      const tokenMatchesAnswerLanguage = (token) => {
+        const n = this.normalizeVocab(token);
+        if (!n) return false;
+        if (direction === 'L2R') {
+          if (allNativeNorm.has(n) && !allTargetNorm.has(n)) return false;
+        } else {
+          if (allTargetNorm.has(n) && !allNativeNorm.has(n)) return false;
+        }
+        return true;
+      };
+
       const allDistractors = [];
       allVocabs.forEach((vocab) => {
         const candidate = vocab[answerField];
         if (!candidate || !candidate.trim()) return;
         const normalized = this.normalizeVocab(candidate);
-        if (!normalizedExcludeSet.has(normalized)) {
+        if (!normalizedExcludeSet.has(normalized) && tokenMatchesAnswerLanguage(candidate)) {
           allDistractors.push(candidate);
         }
       });
@@ -2731,7 +2759,11 @@ export default {
           const candidate = vocab[answerField];
           if (candidate && candidate.trim()) {
             const normalized = this.normalizeVocab(candidate);
-            if (!normalizedExcludeSet.has(normalized) && !options.has(candidate)) {
+            if (
+              !normalizedExcludeSet.has(normalized) &&
+              !options.has(candidate) &&
+              tokenMatchesAnswerLanguage(candidate)
+            ) {
               options.add(candidate);
             }
           }