Sharpteeth speak a different language because they represent a different culture, one of violence and hunting. Just as Romans spoke Latin and Italino, the Barbarians spoke their native dialects such as German and Gaelic. It is possible that the Sharpteeth do not communicate in Leaf Eater Language because a) they do not understand it or b) because Leaf Eater's are their prey, and food source, they don't want to communicate and grow attached like Chomper did. Let's take for example carnivore humans, if someone eats cow, chicken, and pork, they probably wouldn't want to be able to speak Moo, chicken, and Oink, because then the creature that a person is going to feast on could talk to you prior to killing it, in fact this happened on a series on HGTV, a chicken starting screaming "help me!" At least that is how it sound, and let me say, it doesn't make you want to eat chicken when you understand the chicken language.