I've been using Duolingo for Spanish for the past 3 years or so. Around last year I started watching Spanish stuff (TV, movies) and I can usually get about a third of what they are saying, enough to understand what's going on. I'm bad with grammar though so past/present/future tense stuff I'm always using the wrong one. I doubt I'll ever speak it fluently though.
I tried Korean earlier this year but Duolingo is terrible and doesn't really explain it at all, so I think I'll have to look online to get a base understanding of sentence structure and what not.
Yiddish would be fun even though completely useless.