Add native language support in vocab course management

- Introduced a new field for native language in the VocabCourse model to allow learners to specify their native language.
- Updated the VocabService to handle native language during course creation and retrieval, including filtering options.
- Enhanced the database schema to include foreign key constraints for native language.
- Updated frontend components to support native language selection and display in course listings.
- Added internationalization strings for native language features in both German and English.

backend/models/community/vocab_course.js

@@ -27,6 +27,12 @@ VocabCourse.init({
         allowNull: false,
         field: 'language_id'
     },
+    nativeLanguageId: {
+        type: DataTypes.INTEGER,
+        allowNull: true,
+        field: 'native_language_id',
+        comment: 'Muttersprache des Lerners (z.B. Deutsch, Englisch). NULL bedeutet "für alle Sprachen".'
+    },
     difficultyLevel: {
         type: DataTypes.INTEGER,
         allowNull: false,