http://bugs.winehq.org/show_bug.cgi?id=58497
--- Comment #2 from Alexandre Julliard julliard@winehq.org --- (In reply to Ken Sharp from comment #0)
I spoke with someone a few years back (I'm pretty sure it was Alexandre) on IRC (Freenode) but those logs are long gone, or very hard to find. From what I recall the "standard English" chosen was British. This makes sense as en_US.po exists for the US English translations.
Strings in the code should use US English. That's the resource that's used by default when there are no translations.