"There are thousands of volunteers who donated their labour to Duo... Bit by bit all of our work was hidden from us as Duolingo became a publicly-traded company."

They exist, but they often lack the front end polish and they never get the kind of publicity enjoyed by their for-profit peers.

10,000 sentences and OpenWords are others to consider.

Librelingo

Try Language Transfer. It's free and different in how it teaches.

Anki is the best language app I've yet encountered including non-foss alternatives, but I needed to watch their tutorial video to understand the ui. Also it is only a flip-card app, so you might want to complement it with something else.