When DH and I visited mainland USA, we were complimented daily on our great English. We are Australian. One guy, upon hearing we were Australian, said something like, "wow, that's overseas, isn't it? Like Canada?"
I don't know, is there a prevalent belief that only people in America and England speak English? We traveled around quite a bit, so it wasn't just in one place.
I would have thought that people would be vaguely aware of which countries speak the same language as you, if only for potential holiday purposes.
Although, I've actually met British people who have complained of the same problem. Maybe we were just unlucky with the numbers of clueless people we encountered.
We always replied to the compliments with, "thanks, your English isn't too bad either!"