From a shell in 10.4 or 10.5, I was able to do this:
/usr/bin/defaults read NSGlobalDomain AppleLanguages
To get the list of the language preference for that particular machine. This was done so that I could restore it back to that list after changing it with the 'defaults write' command to something else (in order to help automate l10n testing).
Now, along comes OS 10.6, and AppleLanguages is nowhere in any of our defaults domains. I know that I can alter it for each running application by altering their specific property lists...but at the cost of more complexity.
Also, some of the apps I have under test here are installer packages...and It's a real pain to change stuff (like the .plist I'd have to change here) in those without being somewhat destructive; that's why I chose to do it globally in the first place.
Anyways, it'd be great if I could find where they stashed it now...or if they deprecated it (like a zillion other things in OS 10.6) completely.