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FOSS Language Learning
(beehaw.org)
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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.
I have looked at Anki before and I am familiar with it. I have a few decks on there, one for German via Nicos Weg and another for Japanese Kanji Radicals via Wanikani (I think that's what it's called). I suppose re "fringe" languages, it's only as good as the decks, and there might be some out there for them.
Anyway, I feel like Anki alone isn't enough for me. I can sometimes get a little demotivated to study for a day or two and then I'm set back in my studies (or at least I feel that way).