Mormons, if the Book Of Mormon was engraved on gold plates, why does it read in perfect 1611 KJV english?

The plates that were translated into the BOM were supposedly buried in the ground in the year 420ad until 1823. The BOM contains extensive quotations from the KJV but the KJV was not translated until 1611ad. The BOM contains modern phrases and ideas that could not have been known to its author in 420ad. It also speaks of magnets and compes but these did not exist at that time and how could it mention elephants and horses when these animals were not native to America?