Gregory's Server
- LibreLingo, yup
- Anki can work nicely for remembering things, be it words/meanings/translations or whatever else that can be split up into pairs (classic flash cards) or partially blanked out for filling in as a challenge (aka cloze)
- Moodle is used by some actual academic institutions for their courses
None of it is fedi though. Probably a bit too early an adoption phase for that yet.
There are some other tools as well, but for much narrower niches and with lots of "depends".