Thats pretty much the question i wondering about.
It uses a custom-built language called UnrealScript. As far as I know it doesn't support scripting in any other languages.
So i need to learn a whole new language? Or does it have some similarties to any other languages?
I haven't seen it, but it says on the UDK site that java and c++ programmers will be able to jump right in, so it's probably along those lines. Most c like scripting languages are easy to pick up if you're familiar with java or c#.
The Unreal Script Reference is a good place to start: http://udn.epicgames.com/Three/UnrealScriptReference.html