Allow me to chose my language on http://theoldreader.com/
I hate when the system "think" that it better know what I need.
So, your preferred language varies depending on the site you are visiting? That's a pretty exotic use case. You could find or code yourself a browser extension to change accept-encoding on a per-site basis.
Anyway, we are leaving the request open. Let's see how many people actually need this.
If I change the 'accept-language' in my web browser then on the other website I will get a wrong language. It's a global setting.
You can choose - just change the 'accept-language' http header your browser sends: http://hastebin.com/jufuxelihu
You can usually change the order of preferred languages in your browser settings.