This is incorrect. The US is hardly the only country where people consider Sunday to be the first day of the week. Anyone with some understanding of the history of our weekdays does so, and that historical understanding is not limited to Americans.

It's also not true that the US is the only country to use 12h time; many countries do. Including mine. If you need to write time without ambiguity, you use the 24h format, but in everyday use and in cases where context makes it clear what you mean, people use the 12h format.