Just an idea that occurred to me while reading this thread - how about a story where the protagonist somehow ends up alone in a foreign country, so most of the people he meets speak a different language to them, maybe having to pretend to speak the language for some reason. As an example, the protagonist is English and he's stranded in Germany, so while the main text - his first or second person narration - is in english, anyone he meets can only speak German. There's some fun things you could do with that set up, I think, wether you know the secondary language or not. A couple of examples -
* A girl says something to the protagonist which he thinks sounds like a come-on, but he isn't sure - he has to decide what action to take.
* The protagonist needs to try to ask a girl for directions, the player is presented with a few sentences in German as their end-of-chapter-options. Only ones the correct translation, the others will embarrass the protagonist with his poor German, or have him accidentally insult her or something.
* A girl who seems keen on the protagonist asks him something in German, clearly expecting a yes or no answer. Is she asking you to screw her brains out, or to marry her?
So in the first example, if you know no German, you have to guess if she is flirting with the protagonist or insulting him and which option to pick, if you know a little German you might be able to pick out enough words to make an educated guess, if you're fluent, you know more than the protagonist and can cause him to make the smart or stupid choice as it amuses you.
Does that make sense at all?
I don't think it's what mindtheMILF had in mind, the idea just popped into my head and I thought I'd share.