And now I'm thinking of Captain Blood (1988), which was built around using an auxiliary language to talk with aliens. It's not really a language-learning game (at best you're adjusting your use of a pidgin to match the alien you're talking to), but it's one of the few games I can think of that asks the player to use a constructed language to communicate instead of just treating it as a way to hide information.
(Am I still a bit annoyed that Chants had more stupid stealth bits than parts where you have to construct sentences with the languages you've been spending all game learning? Yes.)

