"Mastering the Comanche trade language means all your research items cost less Coin."—In-game description
Comanche Trade Language is a technology in Age of Empires III that can be researched at a Trading Post built on a Comanche settlement. Once researched, it reduces the coin cost of all other technologies by 10%.
Comanche Trade Language is best used with Home City Cards that reduces the coin costs of upgrades or units.