There was a episode of Farscape where everyone's translator microbes stopped working, none of them spoke any of the other's languages.
This seems like an obvious plot for a Star Trek episode, its strange how its never happened.
I once pitched a Voyager
ep in which the UT broke down and suddenly Neelix couldn't talk to anyone. Alas, it didn't sell, but maybe I can recycle it as short story or subplot someday . . . ?