First, truly learning a language takes a huge amount of time, and I really wanted to be able to learn a language, therefore I chose one that was closely related to the two languages I already spoke. Second, it's easy to find resources for French. They are abundant and free, plus I was able to get personalized language training online for a reasonable price. (My Skype lessons are $16 an hour.)

Finally, I wanted a country I'd be happy to visit multiple times, and where there was some international component that could be useful. I've traveled to Morocco and Tunisia, and so many people there speak French at a high level that I pretty much spent the entire trip both times speaking only in French. At the time I was also living close to the border with Quebec, so I occasionally used it there.