But this is circular reasoning. If every web dev ignores the browser setting and uses IP to determine language then the browser setting is irrelevant and no-one

But this is circular reasoning. If every web dev ignores the browser setting and uses IP to determine language then the browser setting is irrelevant and no-one bothers setting it. So the browser setting is inaccurate for the majority of users and the web devs can cheerfully ignore it. I have started a one-man campaign of creating support tickets for all sites that ignore my browser setting. Join me!