May I know if, like in Seoul, there are many Mandarin speaking Koreans around Jeju.
Probably you have only been to places where there are many Koreans capable of speaking Mandarin, like Myeongdong. Koreans in general don't speak Mandarin, and I think it would be similar in Jeju also. In tourist places probably yes, but in general no.
At the shops in Shin-Jeju, many staffs tried to tell me in Mandarin when I first asked them in English. And street roads and attraction sings are written in Korean and Chinese as well. Because tourism is a big matter for Jeju's economy, I had an impression that there are more Chinese signs than in mainland S.Korea. You will meet many Mandarine seaking people all over in Jeju.
I noted that there is at least 1 staff who is able to speak mandarin in most of the shops or restaurants .