The policy for British vs. American English across the entire SE network, as far as I'm aware, is that we ought to honor the spelling (and grammar) of the OP.
IOW, if you wrote in British English, it is an incorrect correction to change your spelling to conform to American English. And vice-versa.
However, we must be gentile... many people aren't aware of the differences, and might think that "evangelisation" (not to mention other differences) is actually a typo.
My suggestion: Roll-back the change, or if it was part of a larger change, simply re-edit the post to revert to your preferred/original spelling/grammar.
If you feel it's a chronic problem, you might mention it in a comment to whomever edited your post, so they know they're making a mistake (probably quite honestly).
I don't think anyone here is on a crusade to convert SE to American (or British) English... I'm sure it's an honest attempt to improve things without realizing it can be controversial.