The part about "accept-language" doesn't address the real problem. If I copy the url to a piece of content that is available in 2 languages and send it to someone else, I want them to read it in the same language as I originally read it. Sadly, I cannot copy Accept-Headers in a fashion that "normal" people understand. To me, resources written in 2 different languages may be instances of the same thing, but not the same resource.

At worst, the page in question uses automatic translation (not so uncommon when it comes to large knowledge bases), so the translation might be odd.

Basically, according to the article, Wikipedia has it all wrong.