Vader means father in Dutch. Sort of. The Dutch word is pronounced quite differently. If you know about things like Grimm’s Law and how languages change, you can easily see the connection between “Vater,” “vader,” and “father.”
Lucas went to school with a guy named Gary Vader. He also brainstormed a lot of names, and the name “Darth Vader” first shows up in one of the early drafts as a fairly random Imperial general. This was long before the idea of making Vader the main villain, let alone Luke’s father, ever came into being. The similarity of the name Vader to the Dutch word for “father” is largely a coincidence.