Mosiah 24:4

and he appointed teachers of the brethren of Amulon, in every land which was possessed by his people: and thus the language of Nephi began to be taught among all the people of the Lamanites.

He made Amulon’s friends teachers in every land his people lived in. This is how all the Lamanites started to learn Nephi’s language.

He appointed Amulon’s brothers as teachers throughout all the territories. Through them, the Nephite language began to spread among all the Lamanite people.