Do the browser defaults in any country include multiple language settings? It seems likely that in most countries, the default would be only one language. And if this is the case, then if multiple alternatives are present in the request headers, it's very likely the user or computer admin has deliberately changed it, and that in turn would mean that sites should respect the choices.

This might still be wrong in when the settings were made by someone other than the current user, or there are multiple languages default-configured, but it might be a step in the right direction.

If web browsers could somehow figure out they were running either under a guest/public-use account, or in a kiosk mode, they could avoid sending an Accept-Language header at all. Then,

1. in cases where the header is sent, it would mean a lot more (and hopefully override both online-profile-stored and IP-detection-based answers);

and 2. in cases where the header isn't sent, using an answer from an online profile setting or IP-detection would no longer be against-standard.

That, and I'd expect public computers to disallow that sort of configuration anyway, so it would be stuck at the default value, which should be sensible for the location it's set up in; it's not like it would move around...