Although not obvious, MDN is a wiki with a pretty level-headed and healthy community. I've done numerous edits that were well-received so I say feel free to change it.
Ah, that is a reasonable suggestion. I would but unfortunately I don't feel technically competent to do so. (I'm more the type of programmer who would rely on MDN rather than write anything on it) It's too bad there's no discussion page like other wikis have. Under the feeback tab in the navbar, there is "report a content problem", but that requires filing a bugzilla. Don't get me wrong, overall I'm quite satisfied with the quality of MDN (it's so much better than w3schools which I was using before), it's just the tone of this particular article.
MDN articles aren't supposed to be just flat descriptions of "Feature X does Y. To use feature X, place the following code in..."
MDN is supposed to be an educational resource that can teach people what different bits of technology are for, when and why they can be used, etc., and so it's appropriate -- if there is a common problem with something -- to mention the problem.
And though I didn't write that article (but I did work on MDN's software platform during my time at Mozilla), I largely agree that Accept-Language simply doesn't work, and some other sort of in-page UI is far more reliable. There's a reason why MDN will do content negotiation, but also shows a drop-down of languages an article is available in and uses language identifiers in URLs, after all.
The description on MDN does fairly accurately describe the state of the world. It could be read as advocacy but I don't think that was the intention, maybe the word "describe" is better than "justify".
So I changed it to:
"MDN attempts to describe why Accept-Language ought to be ignored. The reasons are fairly weak when the..."
Which reframes MDN as a reporter instead of an advocate.
