Why doesn't Duolingo reject translations from people who don't know the target language well?
This seems to me potentially to be one of the more serious problems with the idea of crowd-sourcing translation, and possibly one that the designers didn't fully anticipate. It's obvious that there are very many people using this system to learn (say) Spanish who are not native speakers of English, because they make errors in their English translations that a native speaker wouldn't make. Yet the "vote" of these users in rating the translations of others seems to have equal weight to that of the native speakers, and it looks to me as if the "best translation" is at least sometimes influenced by this. It is certainly the case that the "best translation" fairly often is not something that a native-speaker of English would say. (I'm being kind; there are occasionally "best translations" that are incoherent.)
The best course at the moment would seem to actively browse through translations and downvote incoherent ones that were called correct. When duolingo reaches a point where you can learn Spanish or any other language starting from several languages instead of only English I am sure the problem will decrease if not disappear at all.
@JareeZy: Yes, I think this probably helps, and I do actively browse translations on sentences that I find particularly interesting or difficult. But it doesn't make sense for me to do this all the time, since by and large I don't learn much from doing it. @jcbos: I would disagree that the "owners of the site don't care." In my experience , they seem quite engaged and as responsive as they can be; for example, they read and respond to these question threads sometimes, and also also the Duolingo subreddit (which is worth following for that reason). But I think they may be a bit overwhelmed: I think Duolingo has become very popular and there staff is probably not huge, so they can't immediately fix everything that needs to be fixed. Having said that, I agree completely: there are definitely "correct" English solutions in the Spanish lessons that no native English speaker would ever say. I've kind of given up sending in feedback about these....
I agree with the Dexedge analysis of the problem. Unfortunately I really wouldn't know how to deal with this. I'm afraid the owners of the site don't care, because even in the exercises I find things a native wouldn't say. E.g. when you are learning Spanish, the site teaches you to translate 'the fish swim' into 'los pescados nadan', although 'pescados' is the name for fish as in food. The correct translation would be: 'los peces nadan'. A native Spaniard or Latino wouldn't say 'los pescados nadan'.