There has been a lot of chat about this. In particular if you try to download the ISO you for some unknown reason get EN-GB even if (as in my case) you have EN-US installed as your system language. Don't confuse system language with display language, input language or region by the way - it is the language you originally installed the OS in.

The answer is it doesn't matter. Much.

Originally my system language was French - I changed everything to EN-US but there were still some internal descriptions of devices and so on that liked to be described in French and some badly written software (Lenovo and Cisco in particular) would only install in French as they incorrectly looked at system language.

So I gave up and did fresh install in EN-US at Win 8. The ISO I downloaded for 8.1 based on my product key was for some unknown reason EN-GB. I can boot from it but have not tried installing from it (I used Technet ISO).

Unless someone knows a reason to the contrary I'd stick with EN-US (the most popular) as the system language and then regionalise it from there. Adding a regional language (like EN-GB or Swedish or French or whatever) as the SYSTEM language is at best going to gain you nothing and at worst stop you downloading early previews as happened last time.