I... found... a... MISTAKE!!!! (I feel like Mr.Hammersmith!)


::spoiler alert::


In Making Money, the golems have their own "ancient language". However, back in Feet of Clay, when Vimes discovers a meeting room whose walls are covered in golem writing, the golems apparently "spoke" to each other in understandable Morporkian. (And about something as secret as creating a liberator king, too!)

::waits for someone to attribute it to a seam in the History Monks' patching up of space/time::