It's frustrating, but you're not going to learn this all in one day, or even a month. Nobody should expect anybody to.
It took me years to learn how to use Javascript with PHP and SQL effectively. I'm not a master of any of those three languages by any measure, but I can make them talk to eachother in a manner of speaking.
My advice, and you don't have to act on this, is to prioritise - find out what they want you to act on first, and learn about that. Everything else can be a side project, but focus on one language. Once you are confident with JS / RPG / COBOL, then move on to the next highest priority language.
While doing so, try to make the case for upgrading to a newer system - they are easier to maintain, and so require fewer man-hours, and so cost less overall. They may want to hear the business side of things, rather than the technical side, so far as I understand. But my understanding is incomplete. Find out what it is they want before charging in with these suggestions.