> "A friend who had been learning some language in Duolinguo and then couldn't say a sentence to a native", should be proverbial nowadays.

I tried picking up some German via Duolingo once. I thought it was going great, pretty soon I was up to full sentences. Then one day I realized (because my voice teacher sometimes makes me translate the foreign language songs) that I wasn't learning German sentences, I was learning English sentences substituted with German words. German grammar is completely different. I haven't touched Duolingo since.