You can use it using your browser with Office 365, no ?

https://github.com/PistonDevelopers/hematite

It can also disable animation in the web browser. Some website can see this setting and disable some stuffs.

There is librelingo but, honestly, it’s far behind Duolingo.

The closest software I found is to use a flashcard tool (like AnkiDroid) and then search for an open deck of your language using AnkiWeb.

Doing so you get a similar experience than DuoLingo without the gamification and pretty UI.

Which tool do you use to re-encode everything to opus ?

I tried with ffmpeg and it works but I had many issues with covers.

As always, there is an xkdc about this