feat(lang): add Greek display language (#1605)

* feat(lang): add Greek & Polish display languages

Also:
* rename zh-TW display language to 繁體中文‬
* fix plurality of AvailableLocale type name
* consistently use 'en' as fallback/default language

* revert(lang): remove Polish display language due to lack of translation activity

server/api/themoviedb/index.ts

@@ -399,7 +399,7 @@ class TheMovieDb extends ExternalAPI {
   public getDiscoverTv = async ({
     sortBy = 'popularity.desc',
     page = 1,
-    language = 'en-US',
+    language = 'en',
     firstAirDateGte,
     firstAirDateLte,
     includeEmptyReleaseDate = false,