I saw a clever style sheet implementation this week. It determined the correct page layout size using the xml:lang attribute. If xml:lang is set to anything other then "en" use the International ISO standard A4 size!
US Letter is actually the most common paper size for office use in several other countries, including Canada, Mexico, Bolivia, Colombia, Venezuela, the Philippines, and Chile so there are a few more language codes that should be added for the US Letter size if you are doing business there.