1. American English - Living in a former British colony (Hong Kong), I'm constantly learning the differences between British English and American English. My Scottish friend Colin and I often tackle the insurmountable barrier of a common language. Especially when he gets drunk.
2. I used to be fluent in French, and could probably still manage you dropped me into a Francophone setting.
3. Cantonese - I can now speak enough of this Chinese dialect that people think I speak a lot of it; I order breakfast and they ask about politics, and the only reason I know that is because I hear them say Obama" in the middle of it. But speaking Cantonese is a necessity if you want to experience life outside the expat bubble in Hong Kong, and locals respect you a lot for even making the effort. Most expats don't bother with Chinese at all, and if they do, they speak Mandarin because its easier.