Anyway to list the video's existing languages?

[someguy9]

So that you could check does 'en-us' exist? then use that, if not default to 'en' for example.

[aehlke]

find a solution?

[oscarqht]

You may call GET https://www.googleapis.com/youtube/v3/captions to fetch all available captions of a youtube video. See YouTube Data API documentation.