What language did King Benjamin teach his sons?

Reformed Egyptian

King Benjamin ... had three sons.... And he caused that they should be taught in all the language of his fathers. For ... our father, Lehi ... having been taught in the language of the Egyptians. Mosiah 1:1-4

And now, bbehold, 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. Mormon 9:32