Darth Vader being Luke’s father was a huge surprise. However, in many languages, words which sound like ‘Vader’ (‘Vater : German’, ‘Vader : Dutch (as @Forral said)’ and many others) mean ‘Father’. Granted, these words and ‘Vader’ have different spellings and pronunciations.
Still, Wouldn’t someone who knew any of these languages have thought that Vader was someone’s (not necessarily Luke’s) father?
These words have different spellings, these words have different pronunciations (not even close enough), these words come from different languages - based on this "similarity" there was no reason for anyone to think that Darth Vader was someone's father.
Moreover, George Lucas didn't know himself that Darth Vader was someone's father until starting work on Episode V (source, section Creation and Development) so the name was not intended to be a pun at all.
Per this answer at SF&F ...
According to George Lucas' Rolling Stone interview in 2005:
How did you get the name Darth Vader?
"Darth" is a variation of dark. And "Vader" is a variation of father. So it's basically Dark Father. All the names have history, but sometimes I make mistakes -- Luke was originally going to be called Luke Starkiller, but then I realized that wasn't appropriate for the character. It was appropriate for Anakin, but not his son. I said, "Wait, we can't weigh this down too much -- he's the one that redeems him."
Read the rest of the answer for more information which covers the history of the name and possible intentions.
