You could use an xsession script to start Gnome and set it there. Create the file ~/.xsession and add the this:
Code:
LANG=en_US &
exec gnome-session
Set your session type to "Custom session" in GDM (if that's what you use) and you should log in to a Gnome desktop with en_US as the language. Since $LANG will be inherited by gnome-session and we set it before we launch the session manager your entire environment should be en_US.
Håkan