How to change my phone's firmware nationality?
For Samsung devices (unbranded ones) the answers are pretty simple:


As long as the model is the same (exactly the same, even the characters after the numbers), the ROM is compatible. (Usually the bare models without any special characters after them have more international firmwares).
Yes, it's enough if you flash an international/regional firmware on top of the old one. Sometimes it needs a factory reset but usually rebuilding the dalvik-cache is the case.
It doesn't matter if your device is rooted. You may be able to flash using Mobile ODIN if you are rooted (no PC needed then) but root access is usually not needed if your bootloader is unlocked (usual case).


So in general you can flash any ROM that suits your device model.