"In welchem Fall" = "in what case" (other word order) . "In case what" means something like, "what are you going to do in that case? I have loosely translated that into german to : Was? But that was not accepted. What are your thoughts?
Google translate says: https://translate.google.com/#en/de/in%20case%20what
Im Falle, was
In English, "In case what?" is at best a fragment of a sentence. I might say "In case of fire, use the stairs and not the elevator". Or I might say "In the case of Madison versus Clark, the defendant was found innocent." Or perhaps "In what case would you come to a different conclusion?"
"In case what" klingt falsch.