Like his ancestors, 65-year-old Clayton Long spent his childhood immersed in Navajo culture, greeting fellow clan members with old, breathful Navajo words like "Yá'át'ééh." Then he was sent to an English-only boarding school where his native language, also known as Diné, was banned. "I went into a silent resistance," Long says from his home in Blanding, Utah. He vowed that he would help to preserve it after he left, work he has done for about three decades as a teacher. Long is one of the educators working with language-learning startup Duolingo on the company's latest endeavor: using its popular app to revive threatened languages.