I would like to learn a couple of languages (polish and Japanese - I already learned Spanish to a decent degree) but it seems like every solution is closed source.
I would use books but listening (and speaking) are very important and books won’t help with those.
What would you guys recommend?
Duolingo is useful for wasting time while feeling like you are learning however it isn’t a great tool for learning if that is your goal.
A lot of people reduce Duolingo to their app, but they offer a big website on which you get a lot of explanations
I disagree, of course you can’t learn just using Duolingo, but it is like saying X textbook doesn’t teach you anything because you are only using that. Learning a language is a process that never ends and that requires a lot of different processes, maybe Duolingo doesn’t have absolutely all of them but if you finish a tree of the main languages and some other digging around you will have learnt quite a lot.