As someone who spent years studying and teaching languages, my single biggest advice is to remember that learning a language is not simply a matter of learning words. Don't fall into the trap of throwing a dictionary full of words into an SRS and hoping for the best. Focus on input, not production.

Include phonetic training. Make the user distinguish between each minimal pair in the language, possibly even every single syllable in the language. This is simple, but rarely done.

Include an extensive reading component. This is a monumental task that could be broken into many apps or many short stories that are in app purchases.

Good luck! I'm really, really hoping to see something better than the very low bar hit by existing apps!