Keep in mind that changing the realmlist doesn't work since MoP, as the file doesn't exist anymore. Here's a simple thing I tried with the MoP installation and that worked for me:
Changing your client from EU to US:
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 the line SET locale "enGB" to SET locale "enUS"
Changing your client from EU to US:
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 the line
SET locale "enGB"
to
SET locale "enUS"
Save the file and that's it.