Hi, I think I may have found a bug in the setting of the default locale for new users, but i'm not sure if it's a bug or has been done intentually for some reason :)
In program/include/main.inc, line 84 there is: $_SESSION['user_lang'] = substr($CONFIG['locale_string'], 0, 2);
This prevents the setting of locales which have more than 2 letters in their name (eg, en_GB).
I have changed this line to: $_SESSION['user_lang'] = $CONFIG['locale_string'];
in my version and it seems to be working without issue.
Was there any specific reason that a sub-string of the entire locale was being set? If not, it might be worth making the above change in the next version :)
Cheers, Darren.