FORTRAN, PL/1 and COBOL were not hard to learn. PL/1 was my favorite.
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.
PL1 was such a huge language.
So different from something like C, which is tiny.
But PL1 was easy and it worked.