I also have such situation with my niece. My sister has moved with her husband to Germany and when their little girl was growing they had that issue with teaching both languages at the same time, but it was too complicated for the little girl to learn two languages at once and she got confused every time she was speaking and was telling one word in english and another in german. So I would say that child should be thought one language for now and later on another.
Rick from
http://yourhomeworkhelp.org/physics-homework-help/ .