✨(summary) add dutch and german languages

Based on a request from our European partners, introduce new languages for the
transcription feature. Dutch and German are now supported, which is a great
addition.

It closes #837.

WhisperX is expected to support both languages.

src/frontend/src/locales/de/settings.json

@@ -75,6 +75,8 @@
       "options": {
         "french": "Französisch (fr)",
         "english": "Englisch (en)",
+        "dutch": "Niederländisch (nl)",
+        "german": "Deutsch (de)",
         "auto": "Automatisch"
       }
     }