It is difficult to know exactly what is meant when King Benjamin indicated that the brass plates were written in Egyptian. Perhaps the phrase ’language of the Egyptians’ in [this] verse means the same thing that Nephi meant when he spoke of the language of his father (and thus the language of the Book of Mormon) as consisting of ’the learning of the Jews and the language of the Egyptians’ (1 Nephi 1:2).
That is to say, the Nephite record reflected the Hebrew culture and background of the Jews, but was written in Egyptian characters. In the present context, then, the brass plates may have been records of Hebrew prophets and their prophecies, all recorded in an Egyptian script.“ (Robert L. Millet,”The Brass Plates: An Inspired and Expanded Version of the Old Testament, “ in The Old Testament and the Latter-day Saints, pp. 421-22.)”