Notes on the back of a 400-year-old letter have revealed a previously unknown language once spoken by indigenous peoples of northern Peru, an archaeologist says.

Penned by an unknown Spanish author and lost for four centuries, the battered piece of paper was pulled from the ruins of an ancient Spanish colonial church in 2008.

But a team of scientists and linguists has only recently revealed the importance of the words written on the flip side of the letter.

The early 17th-century author had translated Spanish numbers—uno, dos, tres—and Arabic numerals into a mysterious language never seen by modern scholars....