A good host will redirect to the proper version of the docs when presented with a root URL, but how do you link someone to a specific language, if the Accept-Language is the canonical source for this information?
For example recently I was linked to a Microsoft page via a comment, but it rendered in Japanese, which was a pain to me because I was looking for a specific citation in English. I had to switch language then dig through the site to get back the text the comment author was talking about.
It is nice to try and help a user, but you should always let them override your language decision.
That said, language prefixes should still exist, for debugging if nothing else.