The (roughly) top 20 languages by GDP

There are many reasons to pick a particular language to learn. If there is a de facto second language of importance in your country (Spanish in the States, French in Canada, etc.), it probably makes a lot of sense to choose that language, particularly if you don't see any globe trotting in your future. If your relatives or spouse speak another language, it's not a bad idea to learn that language. And many still choose a language because they like the way it sounds (français, anyone?).

My preferred method of choosing which language to learn is based on its economic utility. As I do tend to be a bit of the globe-trotting type, I've never really limited myself to any region or the like. Without such limitations, it makes a lot of sense to choose the languages you learn based on the percentage of world GDP represented by speakers of those languages.