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Thread.CurrentThread.CurrentCulture = CultureInfo.GetCultureInfo()
call done by EPiServer in the LanguageContext class.
If you look in Admin, Config, Edit Language Codes you can see all the available languages. The Language Key is the one used on the page and in web.config. The locale column is used to look up the culture. To have the default culture on your site be en-GB, change the locale from en-US to en-GB here.
If you need to distinguish between en-US and en-GB you can follow the steps Jeremy Norman outlines in his blog: http://episervernz.blogspot.com/2006/09/in-my-language-please.html
Logically, changing EPsLanguage from EN to EN-GB should give me the correct result if I inspect the Thread.CurrentThread.CurrentCulture before and after Init in my template. It does not. It always give me en-US. Changing the locale of EN in admin does work though. Do you see the same on your system?
/Steve