Q500 4K Q500 ST10+ Controller Firmware US Firmware?

"is there a US firmware for the ST10+ remote control for the Yuneec Q500 4K?"
There is no difference for controller firmware. The only component that has regional firmware is the camera itself.

"What is the difference to the EU firmware?"
None. EU and US Controller Firmware is the same.

"And where can I find these?"
https://www.dropbox.com/s/5oaeki9xh7lq3y7/st10+v01b31c.zip?dl=0
Please note the weak wording of the factory instructions has led to many failed updates and damaged controllers. It has been many years since an update was issued. It is generally recommended to avoid updates if the controller is operating normally and currently has all the functions that you use. If you decide to perform an update, these instructions may provide a better chance of success:
https://www.dropbox.com/s/h1jlowdxev18rze/ST10+ Firmware Update.pdf?dl=0