Per policy, all "rulebooks" (they're called "reference modules" in FG lingo) are read-only and reference manuals are static by nature anyway, so you wouldn't be able to edit that. However, you could make local copies of each class by opening the master Classes window, grab a class and drop it right back into that window. The new copy will be editable so you can change the language there. (That's true of all record types, and that's how all bestiaries are intended to be used by GMs to create encounters.) You could then export (either with the export button or the /export chat command) as a Player Module that your players would be able to load themselves and use that in lieu of the Players Handbook.