The main language spoken on Jeju Island is Korean. However, it is the Jeju dialect of Korean. Just like how some people in the United States talk differently depending on where they live, people speak differently throughout South Korea, too. Examples in English are soda vs. pop, water fountain vs. bubbler and other things like that!
The South Korean won is used on Jeju Island and everywhere else in South Korea. Every one U.S. dollar is equal to around 1,100 Korean won. When you see the price of something in South Korea, you will see this symbol in front of the price: ₩, instead of the U.S. dollar sign, $.
A 12 ounce bottle of water in Souh Korea costs around ₩550, which is around $0.45.
I had some delicious seafood ramen at the beach this weekend. The seafood ramen had crab, fish, clams and other shellfish in it. When I came to South Korea I never thought that I would like eating so much seafood, but I am glad that I do!