Why do demons speak latin?
They speak in Latin or any other language dead or alive. They have been around since the dawn of time and have acquired the knowledge and ability to have mastered all languages. Just think what you can do with Rosetta Stone and a few thousand years, then multiply that knowledge by thousands since demons were created at the same time as angels, before the Jinn, and before mankind. Angels and demons are made from light, the jinn is made from fire, and humans are made from earth. Truth is whatever language you are comfortable with speaking, is the language in which an angel or demon will speak in.