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?
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.