Other than with like a PDF of a textbook (or a physical version thereof), I don’t know many FOSS apps/sites for learning a language whether its a fringe language with few resources like Te Reo Māori, Irish, or Georgian or a major language like German, French, or Spanish. Have y’all seen any sites or apps for this?
I have neither used a dedicated app specifically for learning the language, nor completed “learning” the language to a passable level, but I did start learning japanese using only freely available online resources. There was quite a lot of information in video and text form regarding grammar etc… For training vocabulary and the japanese characters (I only learnt Hiragana and Katakana - no Kanji) I used Anki. In my oppinion it’s very doable to learn a language this way without a course (though keep in mind that your pronunciation will suck if you don’t get feedback from people that actually speak the language) - in the end I was just too lazy to learn vocab.