And that can be "close enough", until you for example serve English speaking people in Ireland the Union Jack. Both languages and flags can be sensitive topics in certain parts of the world.
Or you could display everything in Korean to those with a Korean IP even though in some cases it will happen to be an Austrian who'd be thankful to see a German flag (or any flag!) on the screen to help them change the language. Then they can worry about the currency.
> How will users from these countries react to an English, British or American flag?
I stopped reading right there because no one cares. If you have users that care then provide them with choices, just don't let them get stuck wandering around a page in Russian or Greek or whatever because you thought someone might be offended by a flag. Perhaps they're offended by crappy websites with no obvious way to change the language? I know I am, show me any flag from any part of the former British empire and let me get on with what I was doing.
Last time I fiddled with some automated kiosk at Ben Gurion airport I noticed they used a Jordanian flag for Arabic. That's an interesting choice, because if I had to guess most of the people choosing Arabic at that kiosk would not choose that as their flag.
(The Tibetan flag is literally illegal to display in Tibet, as the Chinese government considers it a symbol of the separatist movement. So you probably don't want to use that...)