Elaborate por flavour.
Like skyrim in French?
Yes.
The vast majority of language learning software that I've seen is basically a glorified vocabulary list.
Being in a class or even better, going there and being part of that culture are generally the best ways to learn.
Like Skyrim in French. Except even better, perhaps even having a school for language in the game.
An idea would be to have the game's language become more difficult very slowly.
Starting out with very simple words, and mostly English and over time transitioning to more complicated language.
(This'd be cool if AI was complete too, being able to have unscripted conversations with NPCs).