Thank you for your answers. :) It’s interesting to see how a game (or any idea) came to be.
I also used Duolingo to learn English, German, Chinese and Arabic.
Coincidence: apart from revising my Spanish, my true current target languages are Russian (the dual of your English :p Although I am French, as a matter of fact XD), German too, Japanese (connected to Chinese through writing), and Hebrew (connected to Arabic). ;) (I feel I really managed to achieve a useful little knowledge of German, especially in reading; Japanese and Hebrew are tougher, I need to train on more material.)
I am curious and keen on amassing knowledge; glad to find a fellow language-oriented person. :)