Many English-speakers who live in China quickly find that any attempts to mix freely with local Chinese people is inevitably followed up by hard-to-refuse invitations to personally tutor their sister's or manager's young child in English. It's easier to draw the line so that these "invitations" don't come.

I know the type of situation you're talking about but it's also a cop-out of an excuse.

Firstly because there are plenty of westerners in China that are perfectly happy getting by completely in English regardless of anything else. 'Not wanting to turn in to a part time English teacher' doesn't account for the large number of westerners who have been in China a few years but still can't communicate in Chinese beyond 'knee how' and 'shay shay'.

Secondly, it's easy enough to say no to those situations, you just need to learn to say it the Chinese way, with a non-committal 'maybe' sometime at some undetermined point in the future.

Finally it's also possible to build a local social circle with people who aren't interested in English - my Chinese improved the most when I did this.