How come the Book of Mormon speaks of a written “language of the fathers” which was not the common language?

Multiple times in the Book of Mormon we are told that there is a “language of the fathers” which is different from the common language. Such as in Mosiah 1:2:

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 were delivered them by the hand of the Lord.

Bruce E. Dale and Brian Dale note how the Maya had particular languages which were only known by the elite and required to be learned by candidates for high office:

King Benjamin “caused that [his sons] should be taught in the language of his fathers, that thereby they might become men of understanding.” Later, his son Mosiah became the ruler of the people. Likewise, Enos (a prince of sorts) was also taught in the “language” of his father. One is led to ask: “Was the regular course of education not sufficient for these young men; was their common language not enough to qualify them to lead?” Apparently not.
– Bruce E. Dale and Brian Dale – Joseph Smith: The World’s Greatest Guesser (A Bayesian Statistical Analysis of Positive and Negative Correspondences between the Book of Mormon and The Maya)

Where would Joseph Smith have got this idea? How would he know that some languages would fade out like this but be kept by the elite?

See:

Bruce E. Dale and Brian Dale – Joseph Smith: The World’s Greatest Guesser (A Bayesian Statistical Analysis of Positive and Negative Correspondences between the Book of Mormon and The Maya)

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