I am unable to recreate this problem.

My Firefox sends the following Accept-Language: "en-us,en;q=0.8,sv-se;q=0.5,sv;q=0.3".

When I go to Google.com in a new private browsing context, I am redirected to Google.se with English language. When I go straight to Google.se in a new private browsing context, the page is in English.