Just a few decades ago, there were no living speakers of Wampanoag, the native tongue of the Cape Cod–based Mashpee Wampanoag tribe. (Their ancestors famously shared a meal with the Pilgrims, 400 years ago.) Then, in 1993, Jessie Little Doe Baird, who was then in her 20s, had a series of dreams about her ancestors, she told Yankee magazine. They were speaking to her, but she couldn’t understand them. A prophecy known to the tribe said that their lost language would come back when they were ready for it, revived by the children of those who had broken the language...