GWT gets locale from either the locale property or the locale query string. If neither is specified, it uses the "default" (ie en_US) locale.
Why doesn't it get it from the browser settings?
It seems the only solution to this is to replace your static html launch page with soemthing like a JSP that reads the browser locales and sets the locale or redirects using the query string. There has to be a better solution than this or simply hard-coding a locale, surely?