Don't know; good luck with that. Wild guess would be: use Intent?
Presumably your app user would only be using the local language anyway, which would already be installed. Perhaps you are solving a problem that only a small percentage of your users will have.
Then again, I have a vague recollection of enjoying a variety of accents when I used TTS six months ago, and I doubt that I installed those language packs or anything, so... I'll have a look at what I did there. But everything was in English, just spoken with the accent of a different language. Or maybe it was of a different country, which is not quite the same thing.