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Every once in a while, I hear about how Darth Vader's name translates to "Darth Father", so those who spoke a foreign language had a clue that he was someone's father.
One high profile example of this is Pitch Perfect, where a character who had never seen The Empire Strikes Back predicts the twist ending:
Beca: [Movie endings] are so predictable, like, the guy gets the girl, and that kid sees dead people, and Darth Vader is Luke's father.
Jesse: Oh, right, so you just happened to guess the biggest cinematic reveal in history?
Beca: "Vader" in German means "father". His name is literally "Darth Father".
Personally, I think that Beca is full of it because "Vader" is a Dutch word, not German (although "Vater" is).
But it did raise the question: prior to the release of The Empire Strikes Back, did anybody actually predict that Vader is Luke's father? Or is this just a case of Monday morning quarterbacking?