A sign language once let 100,000+ people across dozens of tongues trade and negotiate

Plains Indian Sign Language was a shared gestural system used across Great Plains nations speaking mutually unintelligible languages, letting Blackfoot, Cheyenne, Sioux, Kiowa and others handle trade, diplomacy and storytelling. An 1885 estimate put fluent signers above 110,000. Unlike deaf sign languages born within one community, it developed specifically as a lingua franca between hearing speakers of unrelated languages.

— Wikipedia contributors, Plains Indian Sign Language — Wikipedia entry on PISL's history and use