I've fallen behind in my "Sunday School" category--this post will try to catch up a bit. Mosiah is the first book of the original, intended Book of Mormon narrative that we have--1 Nephi through Omni plus the Words of Mormon are really replacement material for the original Book of Lehi material which was lost. Metcalfe's essay The Priority of Mosiah is a detailed summary of the thesis (not original to him) that the material in Mosiah through Moroni was actually dictated before the replacement material in 1 Nephi through Omni. As noted by Metcalfe, that ordering of the text also sheds light on several thematic inconsistencies which otherwise emerge. In addition, inspection of the original manuscripts suggests the original first chapter of Mosiah was lost (recapitulated, one would think, by the summary material in Omni and Words of Mormon) and the present "Mosiah 1" was actually the second chapter.
The first half of "augmented Mosiah" (using Omni and Words of Mormon as the best guide to what would have been in the original first chapter of Mosiah) follows events centering on Mosiah I, Benjamin, and Mosiah II. Mosiah I is depicted as a spiritual translator: "[I]n 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" (Omni 1:20). Compare Omni 1:25, where "the gift of interpreting languages" is noted as a spiritual gift in a short list styled after Paul's list in 1 Corinthians 12. On the other hand, King Benjamin is depicted in Mosiah 1:2 as a sort of wise man and scholar who imparts secular knowledge of ancient languages (which would be Hebrew or Egyptian, one would think) to his sons. The confusion in Mosiah between translation as a spiritual gift and translation using secular knowledge of ancient languages characterizes modern Mormonism as well.
Later, Mosiah II reverts back to spiritual translation, using seer stones: "[H]e translated [indecipherable characters on plates] by the means of those two stones which were fastened into the two rims of a bow. Now these things were prepared from the beginning, and were handed down from generation to generation, for the purpose of interpreting languages" (Mosiah 28:13-14). Earlier, Mosiah II's gift and seer stones are described as "a gift from God, whereby he could interpret such engravings" (Mosiah 21:28).
The possession and use of seer stones is one of the primary themes of Mosiah. That is most evident in Mosiah 8, where a fellow named Ammon explains that Mosiah II "can translate the records" that Limhi's people discovered because Mosiah II possesses "interpreters" but that "no man can look in them except he be commanded" (Mosiah 8:13). Possession of seer stones and the power to use them makes one a seer, and "a seer is a revelator and a prophet also; and a gift which is greater can no man have, except he should possess the power of God, which no man can" (Mosiah 8:16). So one who possesses seer stones is greater than a mere prophet.
The possession and use of "interpreters" or seer stones can also be seen as a primary theme of the Book of Mormon taken as a whole. They are referred to only occasionally in later books (e.g., Alma 37:21). Yet in the impassioned first conclusion of Moroni's completion of his father Mormon's work (Mormon 9:30-37) he clearly alludes to "intepreters" in Mormon 9:34. Later he is more direct, referring to his burying a set along with his writings: "I have sealed up the interpreters, according to the commandment of the Lord" (Ether 4:5). Thus the theme of obtaining and using seer stones stretches the entire length of the Mormon/Moroni narrative, from Mosiah to Ether and Moroni. In light of Ether 4:5, Moroni's comments in Moroni 10:2 about sealing up records would appear to refer to his seer stones or "interpreters" as well. Once again, "the interpretation of languages" appears in Moroni's list of spiritual gifts at Moroni 10:16 as it did earlier in Amaleki's list at Omni 1:25.