I'm not American and yet it works fine. Perhaps you mean non- North American users?
AFAIK, the biggest localization issue is coming up in locales where the decimal separator is not a period and the thousands separator is not a comma.
As for the performance issues on poor internet connections -- well, I think that for a Silverlight app (or a Flash app or a Java applet), it's making way too many round trips back to the server. It's possible they could improve things considerably there at the cost of making the initial download larger.