The two languages I'd love to learn are Arabic and Korean. I've already studied in depth (as opposed to a smattering of everything in linguistics) -- either as majors or for fun/enrichment -- are German, French, Polish, Spanish, and ASL. The first three have been useful in my work, as most of the people from the French IT companies I work with speak those languages -- usually French as a native language, but others are immigrants from Slavic countries or grew up in Alsace with German-speaking parents. Spanish is pretty much a second language here. I'd love to learn Korean because I work with Koreans/kids of Korean parents in the afternoons. And I'd love to learn Arabic because so many of the French I work with are bilingual Arabic and French.