Yes, it's broken in most browsers. For example my system language is set to en-gb which is British English, but my browser reports en-us which is American English. Most websites don't distinguish between these two languages though.

The main reason that Accept-Language is problematic is that it forces the user into using the system language, so a Sweedish person in London at an Internet cafe ends up with websites in English, so there must always be an easy way to change the website language within the browser.

But a browser-wide setting would just annoy bilingual people who want content is different languages depending on context.

And this is the reason why I don't force anyone into viewing a particular version of the site -- I only use it for displaying an opt-out notice guiding them to the version that I think they may prefer.