Just curious, but does his parents speake English at all? If so, that would make it easier for Yao to learn it, being that his mom lives with him. Does Yao have any way of learning English, other than just absorbing it through his travels?
Trust me, the quick way to improve his English is to get rid of his translator and live away from his parents.
Seriously, YM's father said in an interview that his and his wife's Eng is better than those who never learnt. He said he learnt it at school but has already forgot a lot of them. Also YM has said that he read USA Today and some other papers every day to learn Eng.