Thanks for adding Polish :)
I just want to thank you all for making it possible. I've been waiting for this for years. Duo is changing the world :)
Please excuse my cross-posting of a Duolingo-specific memrise vocabulary list I've recently begun putting together: http://www.memrise.com/course/920188/duolingo-polish-checkpoint-1/2/
How did you attain fluency in these three other languages? My mother tongue is Polish as well but it doesn't help me know that the word for potato in Czech is "brambor", while in Polish it's "ziemniak". I can understand some of these languages when spoken but can't speak them at all, because that would require a knowledge of the differences. So if you have any tips, let me know!