I bet if you simply copy the whole MTD5 and MTD6 partitions from a US camera, you'll get it to show up as a US camera. Most likely, these flash partitions are not touched during a firmware upgrade, which is why the language flag is persistent, even after a TFTP flash. And when you use TFTP, it just does as it's told and doesn't check the language flag.
The only issue would be if something else important is contained within these partitions, such as the MAC address of the network adapter(s), a serial number, or perhaps some information related to WIFI on devices that have it. A simple file comparison/differential after dumping these partitions from both a US and Chinese camera may help determine if there is any other useful information stored besides the language flag.
I think CBX mentioned he has RS232 access to the boot loader, which means he should be able to recover any brick this may cause. I would only recommend that someone with that kind of access try this in case I am wrong.
PS - I take no responsibility for anything that goes wrong!