Dwarfish
From Discworld MUD Wiki
Dwarfish is a language spoken by Dwarves. It is not player-learnable and so is only spoken by NPCs. Through the use of certain rituals it can be understood however. In the past the language has also been known as Dwarvish and Dwarven. However the three variations were used inconsistently and all variations were later changed to the current spelling of Dwarfish for consistency.