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.
Oh, come now. This speaks of "multiple" languages? It says no such thing. It implies that some "Lamanites" didn't know the "language of Nephi". But WHY? Because they spoke Latin? or Akkadian?
Nephi,
I make a record in the language of my father, which consists of the learning of the Jews and the language of the Egyptians
Book of Omni,
Behold, it came to pass that Mosiah discovered that the people of Zarahemla, came out from Jerusalem, at the time that Zedekiah, king of Judah, was carried away captive into Babylon. And they journied in the wilderness, and was brought by the hand of the Lord, across the great waters, into the land where Mosiah discovered them; and they had dwelt there from that time forth. And at the time that Mosiah discovered them, they had become exceeding numerous. Nevertheless, they had had many wars and serious contentions, and had fallen by the sword from time to time; and their language had become corrupted; and they had brought no records with them; and they denied the being of their Creator; and Mosiah, nor the people of Mosiah, could not understand them.
But it came to pass that Mosiah caused that they should be taught in his language. And it came to pass that after they were taught in the language of Mosiah, Zarahemla gave a genealogy of his fathers, according to his memory: and they are written, but not in these plates.
And it came to pass that the people of Zarahemla, and of Mosiah, did unite together; and Mosiah was appointed to be their king. And it came to pass in the days of Mosiah, there was a large stone brought unto him, with engravings on it; and he did interpret the engravings, by the gift and power of God.
And they gave an account of one Coriantumr, and the slain of his people. And Coriantumr was discovered by the people of Zarahemla; and he dwelt with them for the space of nine Moons. It also spake a few words concerning his fathers. And his first parents came out from the Tower, at the time the Lord confounded the language of the people; and the severity of the Lord fell upon them, according to his judgments, which is just; and their bones lay scattered in the land northward. The Book of Mormon, 1830, p.150
Book of Mosiah
AND now there was no more contention in all the land of Zarahemla, among all the people which belonged to king Benjamin, so that king Benjamin had continual peace all the remainder of his days. And it came to pass that he had three sons; and he called their names Mosiah, and Helorum, and Helaman. And he caused that they should be taught in all the language of his fathers, that thereby they might become men of understanding; and that they might know concerning the prophecies, which had been spoken by the mouths of their fathers, which was delivered them by the hand of the Lord.--And he also taught them concerning the records which were engraven on the plates of brass, saying, my sons, I would that ye should remember, that were it not for these plates, which contain these records and these commandments, we must have suffered in ignorance, even at this present time, not knowing the mysteries of God: for it were not possible that our father Lehi could have remembered all these things, to have taught them to his children, except it were for the help of these plates: for he having been taught in the language of the Egyptians, therefore he could read these engravings, and teach them to his children, that thereby they could teach them to their children, and so fulfilling the commandments of God, even down to the present time.
Zeniff,
I, ZENIFF, having been taught in all the language of the Nephites, and having had a knowledge of the land of Nephi, or of the land of our father's first inheritance, and I having been sent as a spy among the Lamanites, that I might spy out their forces, that our army might come upon them and destroy them; but when I saw that which was good among them, I was desirous that they should not be destroyed
Like the people of Zarahemla, the language of the Lamanites had probably become "corrupted" from that of the Nephites. That is all. It consistently speaks of a "language of the Nephites" which was tied to Egyptian. There were the group in Zarahemla that spoke some "corrupted" language, but it was discarded and they were taught Nephi's language.