I find Spanish the least helpful language to know, personally. I've always lived in east coast urban US. I'm a native English speaker and know a little Spanish and one of my parents is off-the-boat Colombian (she speaks perfect English and lost a lot of her Spanish since she was a kid, so very little carried over to me). But frankly anywhere I have traveled where Spanish is prevalent, in our out of the US, English and one year of high school Spanish is more than enough to get by.
I basically only ever speak Spanish with native Spanish speakers for fun, very rarely because it was particularly useful. Getting by with English among Spanish speakers is a lot more about being patient and gracious: ask them if they speak English, thank them for speaking English, etc. BTW to my fellow Americans, that applies anywhere.
As an adult who does travel quite a bit and wants to learn a third language, I tried to approach this as an optimization problem. While by population Mandarin is high on the list, it's not very prevalent outside of China. When I realized sheer population wasn't going to be a helpful way to look at it, I started looking at the numbers of countries and both their primary and secondary languages. If you ignore English, it turns out that German, Italian, and French start looking really helpful... not for speaking in Germany, France, or Italy, but in the most number of other places that don't necessarily have a lot of English speakers. I'm opting for French, personally, but I couldn't find any real reason to pick one over the other.
But to the OP's question, if s/he is not planning to travel and is specifically asking about programming, I have never needed anything other than English while programming, ever. At that point I'd say, you clearly have an itch to learn a language, learn whatever language you want to learn.