Chrome sends this:
Accept-Language: en-US,en;q=0.8,en-GB;q=0.6
browsers will let you set multiple languages, in your case it looks to me like you have both en-US and en-GB but en-US is higher in the list so it uses that instead of en-GB as the most preferred language. It may not be obvious in Chrome but the list is sortable you can drag en-GB to the top of the list and it will then be seen as your preferred language and you should see it reverse the order in the accept header it sends, the language on top of the list will always be first.
Hope that helps,
Joe