I just found the problem and the solution:
The reason it works fine with en_AU is because that (and the en_US) date format js files are included in the installation. The no_NO is not, nor is the en_GB that I'm using.
For some reason, earlier versions of FarCry gave you every js file, whereas now they are forcing you to accept Australian, American, or errors.
I can't remember where I got all the original js files, but /farcry/core/webtop/js/dateJS/ should have 157 files in the format date-xx-XX.js - my install only gave me two!
Jorgen, if you can't find any then let me know and I can see if I can find a way to send them all to you...
-Phil