Don't be so hard on English mistakes.
English is not my native language. Some times I make mistakes like forgetting the "s" in the end of verbs for he/she/it, or write "who" instead of "whom". And then I loos a hart. I think this is wrong. Improving my English is not wrong, but at the moment I am trying to learn French. I wish that the program wold not be so hard on me for doing tiny English mistakes that don't change the meaning of the sentence.
It seems like a really bad idea to learn a third language in a second language you haven't yet mastered. I would worry that legitimate language mistakes could too easily get through that way--something I would not want. Ideally it would be better to wait until Duolingo supports your native language. If they did what you propose, it should be a customisable setting.