Not sure if this is what you want...
defaults write NSGlobalDomain AppleLanguages "(en_GB)"
defaults write NSGlobalDomain AppleLocale -string en_GB
That sets the Mac to English GB, and I think it is the keyboard language too. I run it in my user login script, but it can be a script that you push out at login. Replace the en_GB with the one of your choice.
I also use non Apple keyboards, so I have a copy of the set up /Library/Preferences/com.apple.keyboardtype.plist, that I copy into place so that the new keyboard is recognised from the start. If the setting you want is in this list, then you can package it up and push the installer out.
