Implement site content language negotiation

For now, when user goes to site directly to the site, e.g.
http://www.site.com, then site content is shown on default language (same
for all users). Of course later user can choose other language later. There
is such setting in user browser as preferred languages, where he sets up
languages he/she wants to view sites on (by the priority). Such header is
called "Accept-Language" and is sent each time user visits the site. I
propose we detect default user language based on this header. When no
language matches, then we use language, marked as default.