On the W3C QA Weblog - Content Negotiation: why it is useful, and how to make it work:

"A negotiation algorithm trusting a cookie showing that the user has chosen a language different than the one negotiated based on its Accept-Language: header information, and defaulting to Accept-based negotiation in the absence of such a cookie, may be the best of both worlds: negotiated resources, and the guarantee of a consistent user experience regardless of potentially misconfigured browsers."