I was not here, so I can only speak based on someone else's experience.
It appears so much Chinese people has come to Cambodia and settled in PP, they have been able to start business, convienence store, noodle shop, etc. Like it or not, majority of business is owned by Chinese, and it is a fact.
There are a fair number of Khmer at the time, but mostly working for Chinese. The Khmer at the time, seems to be the lower class.
It appears there are little need to speaks Khmer if you are born and raised in PP, whereas in other province, it is not the case.
Yes, if you want to keep the pride to be a Khmer, then you may believe you must speak Khmer. But this isn't the case in the late 60s early 70s.
If are born in Cambodia, then the authorites really have no choice but grand you Cambodia ID/Passport. Your ability to speak Khmer is irrlevant. Besides, Khmer is not exactly easy to learn.
I don't know how many Chinese is in SF or Vancouver, what % are there? If Chinese is ALL YOU NEED to get around, then it is very much Chinatown. I been to SF and Oakland once, this is not the case.