Jeju is a Koreanic language spoken in Jeju Province in South Korea by about 5-10,000 people, all of whom are over 60. It is considered a dialect of Korean by many, include some who speak it, but it differs significantly from other varieties of Korean, and is recognized as a distinct language locally, and by UNESCO as a "critically endangered language".
Jeju is also written Cheju or Jejueo, and in Korean it is known as 제주 방언 [濟州方言] (Jeju bang-eon), 제주 사투리 (Jeju saturi) = "Jeju dialect", 제주어 [濟州語] (Jejueo) = "Jeju language", or as 제주말 (Jejumal) = "Jeju speech".
Jeju has many loanwords from Japanese, Chinese, Manchu and Mongolian. It lacks the formal and honorific language of Korean, but preserves old words no longer used in Korean.
Jeju is taught in one elementary school, and there are occasional radio broadcasts in the language. There is also a magazine in Jeju published by the Jejuo Preservation Society, and some books, newspaper articles and other material.
Information about the Jeju language
Jeju online talking dictionary