Just like any other content management system, you might have stuff (in this case language(s)) that you (or a translator) are working on, but that aren't completed yet, and therefore shouldn't be returned in any API response for your applications / websites to consume since that could result in a website that is just partly translated. It would therefore seem like a really good feature to be able to allow languages to be in different states; at least "Draft" or "Published". If you don't want to go the full-blown CMS way, you could simply have a "Disabled" flag on the language that hides it in the API response.

This would enable you to not have to resort to a "supported languages" list in the application that only shows languages that aren't fully supported yet, which might require you to submit new version to the app store, or release new builds to get the new language out the door (or put the list in some remote configuration parameter such as Firebase Remote Config).