Why do sites regularly place language "en", "fr", etc in the URL rather than leaving it in a session variable/cookie etc?

Surely language is an individual thing, and better set based on the user's browser settings or an explicit setting that they've selected and saved (via session/cookie).

If I'm sending a page I've just read to my french-speaking friend it would be better without any language code in the URL, so it opens in French for him based on his browser setting or on an earlier visit to the site.

This seems very obvious to me... but a LOT of major sites put the language in the URL. So I feel I must be missing something... What?