Add missing languages from Nova-2 supported languages list (#1873)

Followup on #819

Adding more missing languages from
[nova-2](https://developers.deepgram.com/docs/models-languages-overview#nova-2)
supported languages list.

As a mandarin speaker, can confirm, these are different.
    'Chinese (Mandarin, Simplified)': 'zh',
    'Chinese (Mandarin, Traditional)': 'zh-TW',
    'Chinese (Cantonese, Traditional)': 'zh-HK',

Although I'm not a Flemish or German (Swiss) speaker, there seems to be
a difference that is significant enough to impact transcript quality.
source 1 [Dutch and Flemish: What’s the
difference?](https://www.globallanguageservices.co.uk/dutch-and-flemish-whats-the-difference/)
> The most obvious difference when talking with or listening to speakers
of Dutch and Flemish is the pronunciation. Even to the untrained ear,
the two dialects sound very different. While Flemish tends towards
French pronunciations, Dutch in the Netherlands has more of an English
feel.

source 2 [Swiss German vs.
German](https://german-academy-zurich.ch/swiss-german-vs-german-what-is-the-difference/)
> There are also some differences in vocabulary. Let me show you a few

![Swiss-German-vocabulary-examples](https://github.com/user-attachments/assets/67cc457d-63e4-41be-8b54-a2d301ab6b61)

app/lib/providers/home_provider.dart

@@ -20,16 +20,20 @@ class HomeProvider extends ChangeNotifier {
   final Map<String, String> availableLanguages = {
     'Bulgarian': 'bg',
     'Catalan': 'ca',
-    'Chinese': 'zh',
+    'Chinese (Mandarin, Simplified)': 'zh',
+    'Chinese (Mandarin, Traditional)': 'zh-TW',
+    'Chinese (Cantonese, Traditional)': 'zh-HK',
     'Czech': 'cs',
     'Danish': 'da',
     'Dutch': 'nl',
     'English': 'en',
     'English/Spanish': 'multi',
     'Estonian': 'et',
     'Finnish': 'fi',
+    'Flemish': 'nl-BE',
     'French': 'fr',
     'German': 'de',
+    'German (Switzerland)': 'de-CH',
     'Greek': 'el',
     'Hindi': 'hi',
     'Hungarian': 'hu',