I tried the following to change my client, very quick procedure and should work:
Choose one:
- edit your EU client and edit it back after, or
- if you have the room, do what I did: copy the entire installation into another folder (called WoW US or something).
In the copy / in your client open the file
Config.wtf in the
WTF folder in any text editor (I used a code editor because Mac TextEdit screws code up sometimes).
Change two lines (first two lines for me, but they might be somewhere lower in the row of SET blah, depending on what your client wrote last).
From:
SET locale "enGB"
SET installLocale "enGB"
to:
SET locale "enUS"
SET installLocale "enUS"
Save the file and that's it.

Can log into US now. Change it a bit before the meeting though, first time starting up the client might be slightly slower since there are minor differences in the clients that will be fixed.