Peter, I wrote a proc that is called within the request processor and looks into the headers of the user's browser (much like what Google does) and adapts the language for that session accordingly. There are provisions in the proc that it is ignored if the user's preferences cookie has set the language preference into something else.
Last time I checked i18n would check for the package or sitewide parameter that is fixed from the admin. If you think it is interesting I could post it here when I'm back. Furthermore, a parameter could be added so that the administrator could set the policy for her site, i.e. fixed default language or default language from the headers of the browser.
