https://github.com/PistonDevelopers/hematite

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.

As always, there is an xkdc about this