Mosiah 1:2-4 reads as follows:
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 [on the Plates of Nephi, etc.], which were delivered them by the hand of the Lord.
3 And he also taught them concerning the records which were engraven on the plates of brass, saying: My sons, I would that ye should remember that were it not for these plates, which contain these records and these commandments, we must have suffered in ignorance, even at this present time, not knowing the mysteries of God.
4 For it were not possible that our father, Lehi, could have remembered all these things, to have taught them to his children, except it were for the help of these plates; for he having been taught in the language of the Egyptians therefore he could read these engravings, and teach them to his children, that thereby they could teach them to their children, and so fulfilling the commandments of God, even down to this present time.
As generally interpreted, this verse is understood to imply that the Brass Plates were written in the “language of the Egyptians.” However, there are two sets of records mentioned in this passage, the Book of Mormon plates mentioned in 1:2, and the “Plates of Brass” mentioned in 1:3. The question becomes, what is the antecedent of “these plates” in 1:4a? Are they the Book of Mormon Plates? Or are they the Plates of Brass?
I think it is possible to read this passage with the antecedent of “these plates” being not the Plates of Brass, but the Plates of the Book of Mormon. In that case, the Book of Mormon is written in Hebrew language and reformed Egyptian script, and thus it is necessary to know “the language of the Egyptians” in order to read them. If this is correct, then the language of the Brass Plates is not described in this passage, and would presumably be Hebrew.
Another interesting possibility is that the most archaic Joseph (of Egypt) material was in Egyptian, but most of the rest in Hebrew. This would then require knowledge of Egyptian to read the all plates, even if you could read most of them knowing only Hebrew.