Jeju language

The Jeju language (Jeju: 제주말, "Jeju-mal") is a Koreanic language spoken on the South Korean island of Jeju. It is related to Korean.
History.
The Jeju language comes from Middle Korean, which is also the ancestor of the Korean language.
Jeju became a separate language from (mainland) Korean due to its isolation as an island and because in 1629, Jejuans were banned from leaving Jeju, causing more disconnection.
Usage.
UNESCO lists Jeju as a "critically endangered" language, since most of its speakers are over the age of 70. Younger Jejuans speak Korean instead. As of 2010, the language is spoken by 5,000 - 10,000 people, which is less than 2% of Jeju Island's total population.
Dialect or language.
Many linguists think Jeju is its own language, mostly because mainland Koreans cannot understand it. Others think Jeju is a dialect of Korean, although a very different one. The local government calls Jeju as a separate language, and the idea of it being a language is becoming more accepted in both Korean and foreign academia. UNESCO listed Jeju as one of the world's languages in 2011.