automate language choice

status: completed

discussion: automatically change according to browser settings, or have a pulldown menu

info: browser settings are sent in the 'Accept-Language' HTTP header

this will require a modification to mailman, that would use os.environ.get('HTTP_ACCEPT_LANGUAGE') to find out what languages the user prefers, and then it would parse that list and return the first available language, falling back on the default language for the list.