Well I'm Australian, and there were times in the book where i thought about the rest of the world - like when they have to speak to the prime minister.
I suppose the justification would be that if she were to include the rest of the world;' a) there'd be no relevance to the story and b) it would have made the books significantly bigger
and we are seeing the world through harry's eyes... the book could be different if say, we read it as a book called neville longbottom, with his tales about his friend harry potter etc.
could be interesting!