From Discworld MUD Wiki
Non-wizards cannot understand the language, nor can it be learned anywhere else.
Ragnel says: This is the language you must learn to be able to read the spells. Ragnel says: This language is the language spells seem to like arranging themselves in. Ragnel scribbles some thingies on the blackboard. Ragnel drones on and on. You go to sleep in the lecture, but when you wake up you find you seem to have remembered more than you thought you would.
You read page two of the tome: The spell Independent Recurring Vocaliser seems to be written here in spidery writing.
Non-wizards cannot read it.
You read the cover of the closed thin book: You cannot read the spidery writing.