From what I've heard, Google purposely ignores the Accept-Language header because they believe browsers with a mis-configured language (eg. the browser is set to use english when the user doesn't want english) is more common than browsers with a correctly configured language that differs from their geoip location (eg. someone in France who wants to view a site in english).
That was my guess too. I haven't checked if they ignore it for all languages, or just English.
I remember being very confused when another site (wttr.in) would show up in Russian on a friend's computer for some reason. I later learned that clicking "never translate <language>" back then in chrome's prompt to translate would add that language to the Accept-Language header. The site then served this version, despite en-ie and en being higher priority.
