Question: What language did the Jaredites speak?
Answer: In Ether 1:33, the key phrase is “at the time the Lord confounded the language of the people.” In Ether 1:35, the main message is, “The brother of Jared did cry unto the Lord, and the Lord had compassion upon Jared; therefore he did not confound the language of Jared; and Jared and his brother were not confounded.”
These verses are interpreted to mean that the Lord did not change the language of Jared and his people. Since Jared and his people spoke the language of Adam, it is assumed that Jared and his families and friends were blessed to continue to speak and write in the Adamic language.
Source: 400 Questions and Answers About the Book of Mormon by Susan Easton Black, p. 201.