I am native Czech speaker, can I help you with corrections in the Czech course?
I found some mistakes and unnatural sentences...
Apart from what Lrtward said, you could also help out by answering learners' questions in the sentence discussions. While it does not improve the course itself directly, it means the course contributors have more time for the incubator. It will also show your interest, knowledge and patience and prove that you would be a valuable addition as a contributor.
If you have what it takes, show it off in the sentence discussions. Anyone can claim they found problems. Harder to discuss said problems in the proper spot and support your position when potentially presented with evidence to the contrary by the very people who would have to train you and work with you if they decided to accept you as a colleague.
You could apply to be a course contributor in the Incubator: https://incubator.duolingo.com/apply
In the meantime, just report the errors that you find. I believe the Czech course is still in beta testing, so there are bound to be a lot of mistakes and omissions.