Since 1 January 2015, it has been possible to file applications in either Korean or English (before: only in Korean). If you file in English, you must submit a Korean translation within 14 months from the earliest priority date.
For PCT applications entering the national phase in Korea, you must submit a Korean translation within 31 months from the earliest priority date. For PCT applications with an international filing day on or after 1 January 2015, it has been possible to request a one-month extension of the time limit for submitting the Korean translation.