Just changed back to US ATC as I’ve got a project in USA. All I did was replace the en-US.locPak in the fs-base folder with a (renamed) BACKUP which gave me back the original MSFS ATC. Which of course is as near as makes no difference (as far as I know - well I’m a Brit!) to “real” US ATC.
But didn’t the original Mod have an option to revert to the original? Doesn’t seem to be there any more, just an instruction to backup the en-US.locPak file, bit of a backward move methinks.