DaHai,
You need to start socializing with these guys and basically encourage the exclusive use of English rather than Mandarin in both social situations and as much as possible during working hours.
Have had a very similar situation with Chinese speaking guys and you need to enforce the use of english as much as possible. Generally speaking the mainland Chinese guys who have studied english (in mainland China) have a very good
written vocab, but have difficulty getting the words out.....
It is difficult, I've been in China long enough to know better and my language skills are still crap!!
Best of luck.