I'd say the most useful language to learn if you travel a lot after English would be Spanish. You're set for half a continent, and you can get by with Spanish in Brazil. Much better than with English anyway.
As for asian languages, Korean is probably the easiest of the big ones. But in any case learning to speak Mandarin, Cantonese or Japanese isn't that hard. The really hard part is reading and writing.
French has a lot of status but unless you plan to enlist as a mercenary in Africa, it has dubious international relevancy.
