Kinda really late to this, but I would not invest a great deal of time into this if you're looking to create single user/small user games.
Us linguists have studied this for decades, and the harsh reality of language acquisition is that there is no substitute for human contact. Absolutely none. Even anecdotally notice that all the video game success stories were from MMOs for the most part, where people got to interact with native speakers.
If you're interested in this field your basically limited to MMOish type stuff...the moment you remove the human element from a language learning enviroment, you lose at the minimum 95% of it's effectiveness.
XI Wiki

