There's long history between Korean and Japan. The part which is written as Sea of Japan is actually "East Sea" or "Dokdo"
For a long time, people are all around the world was informed here as "East Sea", even Japaneses knew this like that. However, suddenly some conservative Japanese suggested that this sea is theirs, and saying East sea is Sea of Japan with non-reasonable back-ups. This territory is Korea's.
Also, International Hydrogrphic Organization(IHO) decides to use "East Sea" for this area. It means "East Sea" is the international standard name.
You probably are confused because people can insist something with their own historic background.
However, here you should follow the international standards, so you must change the map with corrct information.