Add Vocab Trainer feature with routing, database schema, and translations

- Introduced Vocab Trainer functionality, including new routes for managing languages and chapters.
- Implemented database schema for vocab-related tables to ensure data integrity.
- Updated navigation and UI components to include Vocab Trainer in the social network menu.
- Added translations for Vocab Trainer in both German and English locales, enhancing user accessibility.

frontend/src/i18n/locales/en/general.json

@@ -6,6 +6,12 @@
 	"dataPrivacy": {
 		"title": "Data Privacy Policy"
 	},
+	"general": {
+		"loading": "Loading...",
+		"back": "Back",
+		"cancel": "Cancel",
+		"datetimelong": "dd.MM.yyyy HH:mm:ss"
+	},
 	"message": {
 		"close": "Close"
 	},