Tradetalk is the Issaries cult language. It is spoken throughout Glorantha and is the closest thing to a common tongue Glorantha has. It may be learned almost anywhere, including nonhuman areas, and is the widest known second language.
Tradetalk spread early – it was used by the First and Second Councils to facilitate communication between different species (humans, trolls, elves, dwarves, etc), tribes, and with outsiders. It was embraced by the Jrusteli who used it as the lingua franca of the Middle Sea Empire, and was also used by the Empire of the Wyrms Friends.
It is hard to find places where Tradetalk is unknown. Perhaps in Kothar and Tarien, and in Vormain.
It is a spoken language, but I can’t help but think it has a hand component like Plains Indian Sign Language, which has been documented across speakers of at least 37 spoken languages in twelve families.
Tradetalk is definitely not Esperanto.
This map shows the areas where Plains Indian Sign Language was used in the US and Canada. Source, Wikipedia
