Fixed 23371 -- Changed en_GB first day of the week to Monday.

There is occasional debate over this, but the normal week in the UK
runs Monday to Sunday.

For evidence - here's the calendar of the UK parliament:
http://services.parliament.uk/calendar/

And a couple of support threads suggesting Americans change their
locale to en_GB on Microsoft and Apple in order to get their calendars
to start the week on Monday:

http://answers.microsoft.com/en-us/winphone/forum/wp8-wpemail/how-do-i-change-the-first-day-of-the-week-in/37c27997-1a18-4e51-91b4-fe2b3cd26496
http://forums.macrumors.com/showthread.php?t=1460577
I don't know what problems it might cause, but I would consider this "Backwards Incompatible" not a "Minor Feature". Disagree?