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.
The US Foreign Services Institute releases their learning materials, so if you're okay with a lack of structure I hear they can be very useful, including both reading and listening.
It can be accessed on several different sites. Here's one that came up, but you can find a few more with some searching: https://www.livelingua.com/fsi/
I hear this opinion a lot, and I'd just like to add it manages to help with some exposure and repetition at the very least. I'm sure you're more knowledgeable than myself with learning languages, but I've been using Duolingo to learn Japanese for a little while now and have managed to pick out basic hiragana and katakana here and piece together small words.
I don't have any delusions that I'll be fluent if I finish this course, but if it can help me learn the characters it's worth the time to me.
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.
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.
If you like Duolingo's style, which I honestly do (I mix it with podcasts on the road), then there's probably no close enough alternative.
If you want an open source version because of the ads, just go to your phone settings and set a "private DNS" with dns.adguard.com. There are other options too, but I'll leave them for you to discover. This will filter out most ads on many apps and websites, such as Duolingo.
I get what you mean, but the speaking excersizes involve them recording me speak. That is kinda sketchy in my opinion. It also uses adverts, so transparency is kinda important (again, in my opinion)
Well you can check your privacy policy to see what they do with that. I doubt they can get much personal data from that. Jk they're going to clone you with AI and steal your bank info.