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?
For flashcards/spaced repetition I’ve used Anki quite a bit.
https://github.com/ankitects/anki
I did get good enough at Chinese for a 3-week trip but I only needed to order food and navigate streets.
I have no idea how good it is for “fringe” languages.
As a language professor, I can tell you that 2-hours a week will only maintain your level of a language. Ideally, setting aside time every day is the most important thing.