WINDOW ROCK, Ariz.—Carmelita Lowe stared down at a script inside a makeshift theater on the Navajo reservation and did her best to sound like a cartoon fish speaking in an ancient tribal language.

“Hi, I’m Dory,” Ms. Lowe said softly in Navajo, hoping to fill the role of the chatty regal blue tang in a new Navajo-dubbed version of the animated movie “Finding Nemo.”

The 62-year-old retired Navajo language teacher was one of dozens of tribal members auditioning Friday for voice-over parts in the Navajo edition of the 2003 Disney Pixar blockbuster about a clownfish who embarks on an underwater odyssey in search of his son.

The translation, funded by Walt Disney Studios, is part of an effort by the Navajo Nation Museum to revive interest in Navajo. The language is still spoken by more than half of the Navajo Nation—the largest tribe in the U.S. with more than 300,000 members—but is less popular among a critical segment of the population: children.

Preserving the language is paramount for members of the tribe—so much so that those who can’t speak it are ineligible to serve as tribal president. Earlier this year, a promising young candidate for the position was disqualified due to his lack of fluency, in a dispute that generated strong feelings among members.