Add support for language negotiation

"Language negotiation is a function of the HTTP protocol which lets a server choose among several language versions of a page, based on the URL and on preference information sent by the browser (specifically in the Accept-Language header)." [1]

Currently, http://getgnulinux.org/ always brings the visitor to the default language (e.g. English). Since GetGNULinux.org is a multilingual site, it makes sense to make use of language negotiation. When the preferred languages set in the browser are not available on GetGNULinux.org or the translations for those languages are not completed, the site should load the English version.

Even with language negotiation implemented, the visitor should still be able to get the page in a different language with a selection mechanism. This is to account for the shortcomings of language negotiation [1].

[1] When to use language negotiation. http://www.w3.org/International/questions/qa-when-lang-neg