Ooh this is a good question.

Japanese, Spanish, Mandarin, French, Gaelic (Scottish or Irish).

Japanese because I definitely want to go back to Japan and it would be awesome to know the language. Also watching anime without subtitles would be cool. xD

Mandarin is mostly for convenience; it covers at least portions of a few countries and would be useful for travel. But also in my own country!

Spanish seems like a good choice because it covers a lot of bases. Same with French.

I was gonna go with Dutch because it seemed more useful than Gaelic BUT PEOPLE DO STILL SPEAK IT and I think it's the coolest language. Knowing either the Irish or Scottish variant would be awesome. xD