The interpreters are the Urim and Thummim that the Lord had given to the brother of Jared (Ether 3:23; 4:5). The brother of Jared wrote his vision of the inhabitants of the earth in the Jaredite form of the Adamic language. The Lord knew that no subsequent people would be able to interpret this language without divine help. The Urim and Thummim were designed just for a seer like Mosiah to use to translate the record. Apparently, Mosiah had received the interpreters which were had by the kings. His grandfather (also named Mosiah) had used them to interpret the large stone of the Jaredites (Omni 1:20). The term used when describing translation with the Urim and Thummim is ’by the gift and power of God.’ This is the way we describe the translation of the Book of Mormon, by the gift and power of God (Moroni’s Title Page).
This same Urim and Thummim spoken of in Mosiah 8:13 is the Urim and Thummim that was buried with the gold plates. It was the tool which Joseph Smith used to translate the Book of Mormon—especially early on. He described them as follows, there were two stones in silver bows—and these stones, fastened to a breastplate, constituted what is called the Urim and Thummim—deposited with the plates; and the possession and use of these stones were what constituted “seers” in ancient or former times; and that God had prepared them for the purpose of translating the book. (JS-H 1:35). This Urim and Thummim was taken with the plates by Moroni after Joseph was done translating.
There was at least one more Urim and Thummim because Abraham had one (Abr 3:1) and Moses had one that was placed in the breastplate of judgment (Ex 28:30).
Bruce R. McConkie
“The Hebrew words urim and thummim , both plural, mean lights and perfections. Presumably one of the stones is called Urim and the other Thummim. Ordinarily they are carried in a breastplate over the heart. (Ex. 28:30; Lev. 8:8)…President Joseph Fielding Smith, with reference to the seer stone and the Urim and Thummim, has written: ’ We have been taught since the days of the Prophet that the Urim and Thummim were returned with the plates to the angel. We have no record of the Prophet having the Urim and Thummim after the organization of the Church.’” (Mormon Doctrine, p. 818)