How to configure a language header
The language header has generally been determined beforehand by the language of the operating system, and thereafter by the language in which the browser was installed. If you want to change the Accept-Language afterwards, it can be done deep within the settings of the browser.
Firefox allows you to do this by typing the following in the address bar: about:config and then by searching for intl.accept_languages
Chrome lets you do it by going to Preferences, type language in search box, click Manage languages
Internet Explorer allows the change by going to Tools, Internet Options, on the General tab click Languages, in the Language Preferences dialog click Add
Attention! Please always make a backup of the browser settings before making any major changes!