I should have known if the apps free, you’re the product. Duolingo appears to harvest the most data compared to other language learning apps.
Source: Surfshark Research
Does anybody know some open source language learning alternatives? Preferably Linux or Android
It’s not really comparable. Duolingo is a nice game and all, but if you want to get serious about language learning (having a vocabulary > 10000 words) Anki + example sentences is the way to go.
Your own learning resources + Anki.
However, this requires significantly larger amount of work compared to said learning apps, which has establisged learning course in form of easily digestable chunk.
You need to digest textbook and such on your own into small chunks, then Anki cards. It’s essentially building course by yourself.
Fluent Forever from Gabriel Wymer gives some idea on how to utilize Anki, though the author moved to build his own closed source app.