No one else has reported the error perhaps because they're not experiencing it?
Actually, that's not entirely correct: I have a very similar issue, also since second Mavericks update or thereabouts ... Spell checker in Mail, Safari, Calendar, Contacts, iPhoto, Notes, Reminders refuses to use British English and insists on zeds and such like
No problem in any of non-Apple apps - OpenOffice, Photoshop, etc.
I have even gone and added British English as a default system language (something I have never had to do before - specifying a correct dictionary was always sufficient), and removed US English dictionaries altogether - nope, the problem still here.
My situation is, perhaps, complicated by the fact that, I have US keyboard MBP, but then again, it was never a problem in the last four years: US keyboard; UK regional settings; British English dictionary. Perfect... well, it was.
It is a minor irritation, but one nonetheless. Oh, and I do know how to set regional preferences, etc correctly ;D
I went from this:
to this:
That didn't help, then I deleted the US dictionaries (I did keep Apple one, but it is not wise to remove it):
Like I said, settings are correct:
But, I still get British spelling underlined as misspelled:
Even though, calling up any of the dictionaries clearly displays the British version as the main one:
Puzzling ... And my apologies for excessively pictorial narrative.
I too keep the flag (US) in my top menu because I also use other languages, but I have not seen it revert from my initial setting (US English). I'm not sure which preference file controls the language and locale settings and if I find it, I'll post back.
I've replaced the flag icon(s) with letters, so that it is visually consistent with the rest of Menu bar, but my UK icon is, in fact just a standard US layout:
I too would be very interested, if you can find that file - perhaps then I can fix this...
Just tell me when! 
