One of my best friends has studied Japanese for several years; I can count to ten in Japanese, but not much more. I've studied French and Arabic, neither of which she speaks at all.

Naturally, when we lived together one summer, we fell into the habit of having multilingual conversations. It really is amazing what one can pick up from context and body language and vocal inflection, particularly if you know the other person well.

(The bad part of this is that now all it takes is one non-English word to throw us both into that pattern, and while we understand each other perfectly it confuses the heck out of everybody around us.)