[quant18]

You could try learning something really obscure. As far as I know I'm the only hacker within 1000 miles who speaks any Mongolian as well as fluent English. Someday, two buyers are gonna show up in that market and keep bidding against each other till my rates go to the moon! Dreaming about that day is much more fun than trying to compete with tens of thousands of (French|Chinese|Spanish|Russian)-English bilingual programmers. In the mean time, I try to hang out at a webforum from time to time and answer random tech questions in Mongolian, which makes me feel all warm and fuzzy. (Or less charitably: I can't sell my skills, so I give them away for free).

To be perfectly serious, the recommendation for what language is "useful" to learn really depends on your own language learning skill --- which is a combination of inborn talent, knowing what learning methodologies suit you, and experience with compensating for a bad environment (e.g. I assume you live in the U.S. and attend an Anglophone workplace everyday).

Spanish is a pretty typical answer to this question not because it's the "most useful" but because it's not that hard for English speakers to learn, it's more widespread in the U.S. than other foreign languages, and it even has some applicability in tech (a lot of people I've seen suggesting moving to Latin America to cut your cost of living while you bootstrap your startup). Chinese/Japanese is somewhat harder to speak, a lot harder to read, a lot harder to get a visa to the country as a one-man startup, etc.