Cool idea! I like that the data is pull-requestable. Have you thought about Duolingo style comments, where users can add comments for clarification/context? I find this very useful to get a better understanding of why something is (i.e. "why use "kein" instead of "nicht" here?")

Also, how well does it work on a phone? I'm assuming it's a PWA...?

Also, is there a way to "favorite" courses? From a UX perspective, "My Courses" feels like it should be "courses that I am currently learning". I don't expect that to mean "courses that I have created", especially since a small minority of users will actually do that.