Once you know one programming language, moving to another is usually pretty easy. It should only take someone skilled in the more modern languages a week to learn COBOL. The hardest part might be doing without all the Object Oriented language features they regularly use.
The hardest languages to learn were Lisp and (especially) Prolog, because they are so fundamentally different from the others.
If you want to make a recent computer science graduate mad, tell them that OO is just regular programming in a different pair of pants - cargo pants to carry around all the baggage of a class.