If I need to call a Chinese airline call centre I usually call in the evening UK time, as the call centre is quiet for their mainland customer base. But when I have called at times when the mainland is awake the wait times have been very reasonable or none at all. ​​

The proficiency of the English language of the agents can be variable but it's usually good or good enough. Make sure you pronounce things slowly and clearly and use basic words, if you feel something is getting lost in translation just say "no changes please" and hang up, go have a cuppa tea and call later.