Not sure which one, probably one of the Germanic ones because they are pretty influential in Business and in Northern Europe, some Germanic languages are: English, German, Dutch, Swedish, Danish, etc. but I think I'd learn Frisian because although not many people speak it, I feel it'd still be nice to learn because it is from the same origins as English and I really admire the Frisian culture and people too.
I started learning Latin in middle school, added German and then French (yes, I took all three languages at once) in High School. In college I took Mandarin Chinese (the language spoken by most of the people in the world, actually). I've also studied Russian, and can translate most Spanish and Italian because of the Latin. During high school, I had to do an oral translation from French to English one day. I did it, but when I looked at the class, they all looked shocked. My teacher told me I'd done the translation perfectly, except had translated not from French to English, but French to Latin. Oops. I still got an A for the translation, but had to do a written French to English translation, too, to be sure I could actually do that translation.