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?
There is librelingo but, honestly, it’s far behind Duolingo.
The closest software I found is to use a flashcard tool (like AnkiDroid) and then search for an open deck of your language using AnkiWeb.
Doing so you get a similar experience than DuoLingo without the gamification and pretty UI.
This. Lots of people use Anki for this purpose. When you get used to it, there is no way back.