As I discuss on my page about written translation, it is always best to use a native speaker for written translation. Much of what I discuss on that page is very relevant here. While interpreting it is common for an interpreter to go back and forth from English or Chinese translating into their non-native language. However, because of the nature of written materials (such as advertising, legal documents such as contracts etc.), the fact that they will normally be highly scrutinized, it is best to use someone who is a native speaker of the language into which you are translating. I use an excellent, yet not too expensive, local translator for this. Of course I can read everything he produces and thereby ensure that you get what you expected.