Add missing languages (#819)

Added missing languages from
[nova-2](https://developers.deepgram.com/docs/models-languages-overview#nova-2).
I need Romanian for myself.

Should I also add these ones?

- Chinese (Mandarin, Simplified): zh, zh-CN, zh-Hans
- Chinese (Mandarin, Traditional): zh-TW, zh-Hant
- Flemish: nl-BE
- German (Switzerland): de-CH
- Multilingual (Spanish + English): multi

app/lib/pages/settings/widgets.dart

@@ -11,6 +11,7 @@ final Map<String, String> availableLanguages = {
   'Danish': 'da',
   'Dutch': 'nl',
   'English': 'en',
+  'Estonian': 'et',
   'Finnish': 'fi',
   'French': 'fr',
   'German': 'de',
@@ -27,7 +28,9 @@ final Map<String, String> availableLanguages = {
   'Norwegian': 'no',
   'Polish': 'pl',
   'Portuguese': 'pt',
+  'Romanian': 'ro',
   'Russian': 'ru',
+  'Slovak': 'sk',
   'Spanish': 'es',
   'Swedish': 'sv',
   'Thai': 'th',