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

overseerr-api.yml

@@ -4536,7 +4536,7 @@ paths:
           name: language
           schema:
             type: string
-            example: en-US
+            example: en
       responses:
         '200':
           description: TV details