My group has trouble getting into character and RPing (instead seeming to see the entire thing as a video game-type RPG, focusing on stats and combat and whatnot over character) and in an effort to try to foster some more interactions, for a one-shot I once made a Ninja that could barely speak Common (basically single word phrases only) and instead spoke Japanese. Avoided talking out of character unless it was absolutely necessary.

It made them act in character just a little more (even if it was just for them to ask "What is the ninja trying to say?"). Group was sidetracked by ordering food and watching World War 2 movies, and the oneshot was never actually finished. Despite my pre-school level of Japanese competency, it was good vocab and grammar practice. 

Overall: Success. Except for the game. It was superseded by Generation War. Oh well. Will have to try more methods.