I'm from Argentina, english is not my natural language, but I decide to join this amazing project because I think i'm improving my english meantime learn German. is this correct? or this will affect my learning curve?
Dale che :) I'm a native English speaker and have tried learning German in Spanish :) I love the idea but remember that your English translations will unfortunately be much worse than native English speakers. I see a lot of very awkward, grammatically incorrect English translations, obviously done by non-native speakers, so the goal of Duolingo, to translate web pages, is negatively affected. Hopefully Duolingo will eventually come up with some solutions, like more learning directions (translate German into Spanish, English into Spanish, etc.) in the future. In fact, I think I now see on the front page an option for Spanish speakers to learn English, you might check that out.
I agree with wildnis, I have seen some terrible English translations obviously done by non-native speakers. You can find out the meanings of the words in a sentence by hovering the cursor over them, but that doesn't mean that you will be able to produce an idiomatic translation of the sentence. I don't think you should be doing translations unless you are an advanced or native speaker of at least one of the languages. I think this messes up the system.
I have improved my Spanish by studying German in Spanish (free via http://www.dw.de/dw/0,,2547,00.html) so I think it is a great idea. However, sometimes you need to give your brain a rest and just concentrate on one language for a while. I agree with wildnes and tutto: you translating to English is like Messi playing basketball