Realistically? Probably Spanish. I have several bilingual friends who speak English/Spanish and I think it would be really funny to learn Spanish without telling them, and use it to prank/jump-scare them one day.
Unrealistically? Japanese . . . I'm not one of those weird nerds I promise, I just listen to a lot of Japanese music and it'd be nice to not have to look up lyrics to make sure the music is clean. I also do watch anime, but I'll be honest I really like subtitles so it's not like anime makes me want to learn the language, y'know?