Duolingo is funny site to learn a language , especially for the beginners.
This site might not give us the superpower to learn a language very well, but it helps us at least to know the basic structures of a language .
I used to work with Duolingo in order to learn italian. It was very interesting.I still keep Duolingo in my phone. I think it helps. But even to work with Duolingo you must know elementary knowledge about language which you want to learn.