This would be great if you wanted to talk to yourself without anyone understanding what you’re saying, for example, or if you didn’t want anybody to know what you were writing down. It might also be useful if you enjoy lots of foreign language media (see music, tv shows, movies) and want to experience the “original” rather than through dubs and subs.
Personally, I would go for either Russian or Estonian, both of which are because I frequently listen to lots of music that are in those languages, and they aren’t spoken by any of my friends.
Would probably go for Japanese since I watch too much anime and Japanese media so learning Japanese would be super useful.
But then most anime and tons of Japanese media is translated anyway, except for music, but in that case I listen to a lot more of German music than I do Japanese, so maybe I would learn German instead.
But then, I can probably learn German much easier the traditional way that I can an Asian language, so maybe I should use the cheat to learn the language that would be harder for me to learn traditionally than just German. (Even though I’ve watched so much anime by now that I effectively can understand Japanese much better than I do German.)