Absolutely agreed. The language of the browser is the only thing that should matter. I constantly have issues with this as I want to browse everything in English, my operating system is in English, my browser is in English, but because I have my region set to Polish in the operating system(to get things like comma as a decimal point for numbers and sensible date format) but of course websites assume that I want to see websites in Polish thanks to this region. Just like....stop doing that.
For sensible date and time formats in English, you can use en_GB. I don't think there's any standard English locale with comma as the decimal separator, although my Android phone and Ubuntu installation offer en_DK for some reason.
Linux's locale allows more granular settings, but that's not possible over HTTP.
