General questions about quality of translations
I just attempted to "test out of" the first two big blocks of Spanish. I've lived in Mexico for 5 years and am quite fluent in Spanish. In the space of about 15 minutes I "missed" 6 or 7 translations where my responses were either an alternative but still correct response (in some cases, much closer to native usage than the webpage's poor suggested response), or correct, with the website being incorrect. Who proofs these? Why are there so many errors in basic Spanish? I was actually interested in using this website to learn German but thought I'd try it out with Spanish first, but if there are this many errors in the Spanish section I can't imagine that the German section is even very usable.
I have used both the Spanish section and the French section. In both cases, most of the material in the lessons was review for me. I had the same experience as you did when I first tried to test out of most of the French lessons.
There are a significant number of errors (including completely unnatural translations, and not accepting reasonable alternate translations) in both sections. If any proofreading was done, it was perfunctory. I think duoLingo has been depending on users for improvement of the lesson quality (via messages submitted in response to being incorrectly "marked wrong"). It's even possible that they started in some cases with machine translations, although most of the translations are not that bad.
I am also hesitant about using duoLingo to learn a new language from scratch for the same reason.