It is not made absolutely clear in the Book of Mormon whether Moroni made his abridgment of the record of Ether from Mosiah’s earlier translation (see Mosiah 28:1-20) or whether Moroni took his account directly from the plates of Ether—in which case he would have needed to translate the record as well as abridge it. Sidney B. Sperry has suggested the following concerning this question:
From Ether 1:2 one naturally assumes that Moroni made his abridgment directly from the plates themselves. If he did so, we are driven to the conclusion that it was necessary for him to find his way into the hill Cumorah, where his father had hidden them. Inasmuch as the language of the plates was that of the Jaredite people, it would have been incumbent upon Moroni to translate them by means of the holy “interpreters” or Urim and Thummim before he could abridge them. This would have been a tremendous task, because Moroni says (Ether 15:33) that he had not written the hundredth part of the record, and as it is we have fifteen chapters or about thirty-one and one-half printed pages in our current edition. It seems much more reasonable—for the writer at least—to believe that Moroni abridged the translation of the Book of Ether which had been made many hundreds of years before by king Mosiah. (Mosiah 28:1-20) This translation would also have been available to Moroni in the hill. (Ibid., pp. 346-47.)