I learned French because it is a beautiful language and I wanted to be able to read French literature and poetry in the original.
Also, I have also translated French literature into English (or American, if you like) in order to make it accessible to a wider audience. How is that not useful? I am also currently translating several documents for an asylum seeker from Togo, and have done the same from asylum seekers in Congo and other African countries. This is pretty useful, as being granted asylum will allow them to stay in the US and not return to their home countries where they may be killed for political reasons.
I've also learned Russian and Spanish. I use my Spanish a lot too, but Russian not nearly as much. I adore it though - it's probably my favorite language that I've studied. I'm also learning Irish... and it's not useless either, because it makes me feel connected to my Irish heritage and also helps preserve a language that is in danger of dying out (thanks in no small part to the British... don't get me wrong, I don't hold a grudge and I have absolutely no problem with the British, but... I'm not sayin', I'm just sayin').
In short, I don't believe it is useless to learn ANY language.