Simon Prentis was Zappa's "Semantic Scrutinizer" - translator, in other words. Zappa, stated that Prentis is the only translator working on the numerous foreign language versions of his material that he completely trusted. Gail and Frank asked Simon to take over as translator after they found out that a lot of the earlier translations were wrong.
Simon Prentis is one of the people receiving thanks for their "very special but no less significant contributions" in The Yellow Shark liner notes.  He was also mentioned and thanked in the liner notes of The MOFO Project/Object (2006) album.