I have a 220VAC window air conditioning unit and I would like to shorten the cord considerably. Is it safe to use a butt splice on 220VAC? I think the cord is 12 gauge.

I would NOT splice it. What you can do is cut the end off and install a new plug at the length that is good for you.

The plug from the factory is probably molded on. You can still cut it off and replace it with a screwed on plug.

You could also open the unit, disconnect the cord from the terminals, cut it to length, and re-terminate. You will need a special "crimper" tool for this.