I am a language teacher and see lots of potential for DuoLingo. But the translations people produce tend to sound very stilted and word-for-word. You might want to post a reminder on each translation page: "Remember to make your sentences sound natural and fluent in English".
Agreed, amichail brought up a great concept of not only rating the translation itself but the English outcome as well. The idea would be to rate not only on understanding the spanish word but also understanding how to put the concept of the sentence into a coherent English sentence.
I totally agree here - the problem is one of lack of understanding the pragmatic force of the discourse - underanding the words but not the concepts being expressed. I think that this could be made much better by contextualizing the language more - not just translating sentences but by showing the whole text at some point. This would be helped again by putting the original web page more in evidence - being able to see the original context will help more. At the moment the original page is a small link at the top.
It's also worth noting that some sentences are very difficult to translate fluently and directly. For example, the Spanish pages that I've translated can have run-on sentences that go on and on and on - as a native English speaker, I'm torn between whether to split this into multiple sentences, or do some rearranging.
Splitting up a single Spanish sentence into multiple English sentences, although it makes the meaning better, it makes it much more confusing for a learner to understand exactly how one part of the sentence corresponds to another part of the sentence.
Agreed! Today is my first day on Duolingo and I tried to translate not verbatim but how it would be written in English. However, I found myself somehow downgraded for not matching the bot's translation, even when I was the only person to have translated a phrase. I even found a phrase today that made absolutely no sense and emailed it to my native-Spanish-speaking friends. They, too, couldn't understand it and said it was either grammatically incorrect or was written in a way that no one spoke. I extrapolated a translation based on the context and what I could make out of the sentence.