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?
Comprehensible input, lots of it. Classes, grammar books ,flash cards, and the like are useful supplements, but only a lot of time with the language works in the long run.