In Mormon 9:32, Moroni explains that the plates are written in reformed Egyptian:
And now, behold, we have written this record according to our knowledge, in the characters which are called among us the reformed Egyptian, being handed down and altered by us, according to our manner of speech.
It is important to note that “reformed Egyptian”appears to be a descriptive phrase rather than the name of the language.
Since the publication of the Book of Mormon, scholars have discovered multiple examples of modified or “reformed” Egyptian and even modified Egyptian mixed with a Semitic language, most of which has been found to be written on copper plates.
Why would Joseph Smith mention this in the Book of Mormon? How would Joseph have known anything about Egyptian or modified versions of it?