Actually, I can tell you my experience briefly. You know, I'm also from China. And I have been in Germany for many years. At the first stage, English and German could always mix together in your head. Then, second stage, English could help you to learn German. Third stage, it is very hard to change from German speaking to English speaking, for example, if you have spoken German all the day, and an American talk with you in English, then your brain always give you German words rather than English words. And, the fourth one, no idea. I'm not so far yet. All here I mean is the spoken languages. As for reading and writing, English is always easier than German, at least to me.