“And he [Benjamin] 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. 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. I say unto you, my sons, were it not for these things, which have been kept and preserved by the hand of God, that we might read and understand of his mysteries, and have his commandments always before our eyes, that even our fathers would have dwindled in unbelief, and we should have been like unto our brethren, the Lamanites, who know nothing concerning these things, or even do not believe them when they are taught them, because of the traditions of their fathers, which are not correct.” (Mosiah 1:3-5).
From the above verses it is clear that the the brass plates were written in “the language of the Egyptians” (the “language of the Egyptians”, however, is a broad and ambiguous statement, which I will not address here). This is why the gold plates were also in Egyptian – their model was the brass plates, which were in that language (some form of Egyptian language). Why were sacred texts of the Jews in this language? It comes down to the fact that it was more efficient than Hebrew was for writing with limited space. Making golden plates appropriate for archival writing was also a difficult process. In essence, the script that they wrote in was a type of shorthand and one patterned off the language on the plates of brass (but likely changed over time). It’s also likely that the writing was an Egyptian script transliteration of Hebrew (or some variant of Hebrew over time).