I'm checking the exact same routing, HKG-EWR-HKG just changing the version of the website. It's not a case of originating in the US vs originating in China.

If I select the US site and then try to buy the ticket with my Chinese credit card, I know that it will force me to pay the fare in CNY but I want to know whether it will just convert that K fare from USD to CNY or force me to buy the higher fare class.

Well I decided to try it to see what will happen. Probably would have had to take advantage of the 24 hour cancellation policy if it had worked. It didn't work. After I entered all the payment info I got this message.

Error

We’re sorry, but united.com was unable to complete your request. Please try later or contact united.com Support in the U.S. and Canada at 1-888-491-6573; the U.K., phone 0800 028 5003; elsewhere use the local phone number.

It looks like you can also use Paypal so I guess I can just choose PayPal and use my credit card via Paypal.

That's really strange. I wonder why they don't want Chinese people buying the k fare on that route.

I tried choosing a few other countries to see what would happen. If I choose Canada then I get the cheap k fare in CDN. If I choose Hong Kong then I get the expensive L fare in HKD. If I choose UK then I get the expensive L fare in GBP.

This must be a mistake on the website. Surely United is not pricing fares based on which country issued your credit card. To be clear, I am talking about the exact same flights, only changing the country selection at the top of the page. In that selection page the instruction is "Please select your location or where you receive credit card billing statements to see pricing in your local currency."

Is it correct that certain fare classes are not supposed to be available in certain currencies? For example the HKG-EWR k fare is only available in USD and CDN? If you want to pay in another currency then you need to buy an L fare for double the price?