when you add a language, it adds it in addition to the other languages. From the language screen, tap and hold to change individual settings.
There's not really anything that would "revert" since you can have any number of languages... is it just not keeping the new one? Which setting are you looking to change: phone/windows language, date/time/currency formats, speech/Cortana, or keyboard?
Also if your phone is locked to a carrier, the setting might revert. The hardware defaults to the country it was built for (which is part of the firmware when the phone is flashed at the factory).
One last note, English (United States) has more Cortana features than any other language. The other English variants might not have all the same Cortana features. So if you use Cortana and don't mind the minor differences, leave it as US.