Oswaldo Vidal Martín always wears the same thing to court: a striped overshirt, its wide collar and cuffs woven with geometric patterns and flowers. His pants are cherry red, with white stripes. Martín is Guatemalan and works as a court interpreter, so clerks generally assume that he is there to translate for Spanish speakers. But any Guatemalan who sees his clothing, which is called traje típico, knows that Martín is indigenous. “My Spanish is more conversational,” Martín told me. “I still have some difficulties with it.” He interprets English for migrants who speak his mother tongue, a Mayan language called Mam.

Martín, who came to the United States with his parents in 1999, when he was four, was studying to be an engineer when the trickle of Mam speakers migrating to the Oakland area, where he lives, turned into a flood. In 2014, some sixty thousand unaccompanied minors crossed into the United States,...