[pornel]

Sorry, when I implement language negotiation I interpret "en-US" as lack of preference.

The problem is that en-US is the default and I can't tell difference between user not setting language and user choosing en-US.

Add "en" or even "en-GB" to your Accept-Language header.

[Eiwatah4]

It isn't the default for most people. Download a browser and OS localized to German, French, or British English and Accept-Language defaults to that instead of "en-US".