The thing for 'sounding like (whatever)' is, a chance to practise. The thing that Duolingo does not provide is the live with real people forum. (Well it does, sort of, but it would feel like sending FB 'friend requests' to strangers.) So I'm not surprised that DL learners sound like three-year-olds, I'm actually impressed that there's the courage to speak at all. Give them a couple of beers, that might help.