I've used Duolingo, but I'm afraid I'm completely burnt out on it. Like you've noticed, it likes to test you on words you haven't seen yet. It doesn't like to explain things, and at least with one language it had at least one lesson about a subject that I didn't care learning about at all (must have been sports or something).

I feel that with Polish I had great success starting with a basic textbook, then just reading and looking stuff up as I come across it. Of course, there's issues even with that in Welsh...