count down

Since Herman Oberth was acting as an advisor on this and, the film-makers were helping funding a real experimental rocket launch by the German rocket group for publicity reasons there was clearly a lot of influence in both directions.
As to who invented the countdown, well who knows. Perhaps Lang did indeed invent it as claimed and the rocket engineers like the young Wearner Von Braun later borrowed it.
Or maybe Lang just got it from the engineers.
One things for sure, this is a very technically accurate film for 1929.
It wouldn`t have looked out of place if it had been made in the 1950s.

"Any plan that involves loosing your hat is a BAD plan.""