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

Same. Plus, on local network, everything stay on the network.

I’m starting watch a movie on my PC, then move the movie file into a synced folder, na finish the movie 2" later on my phone.

GNOME, for sure. It works out of the box, and it’s kind of pretty out of the box.

I also tried it on a touch screen PX and it works surprisingly well.