While I was taking up my internship at InfoMan, I created a cooperative loan system for them. I developed the software using a proprietary business language they use for their clients - Progress Open Edge Advanced Business Language.
I self-studied the language first but had a difficult time gathering resources and tutorials as the language is proprietary.
I swear it is one of the most horrifying languages ever built in the history of the information age. Learning it makes you want to pull your hair out and realizing it’s not worth it.