German term (edited): jeder f�nfzigste

It does depend rather on exactly what you are trying to say, so more context would help.

I think either of the versions you suggest is fine, but if you are emphasising the statistic I would prefer "one.... in fifty", e.g.
"only one new truck in fifty had brakes that worked".

To say "every fiftieth" sounds quite a lot more specific, as though it refers litterally to every fiftieth vehicle, rather than taking an average from a large number. "Every fiftieth truck was red" has a quite different implication from "one truck in fifty was red".