Well I know that XML is the easiest, soo if you can probably do that, then yor off to a good start! Then tacle something like python but if you really want to get the most out of modding, you might want to learn C++. Personally, I would learn C++ if I had Time, I learned some small stuff, like how to make words appear o screen in dos window, but thats it and to mee its a COOL language and seems fun to learn. Maybe im a nerd that likes math, but i would try to learn C++ to use the SDK and its good for modding other games of your preference.
EDIT::::: DANG, The Great Apple YOU BEAT ME TO IT!!