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?
There are a couple of language learning tools I know of that are FOSS, the first is of course anki, which you know of, the other two are both focused around reading, specifically Lingq like tools:
If you’re not familiar with how Lingq (and the above two lingq-inspired tools), you provide it content and then it highlights unknown words, you can click the words which show you what they mean and you can then add it to a set of learning words. Over time through these words coming up naturally, you learn them. I’ve found it just pretty much lets you more easily read interesting texts earlier, because looking up words is far easier.
I will definitely be considering LWT when I look for apps to self-host once I move out of my current place (long story). Might even look into developing a better UI if I get the time.