Using the proper name for the body of water between Korea and Japan is not simply a question of naming a geographic area but is part of a national effort by the Korean Republic to erase the legacy of Japanese Imperialism and to redress the unfairness that has been resulted from that period. The way African-Americans were enslaved, so was the Korean people.
Therefore, I urge you to use 'East Sea' to describe the body of water in question or to use both Korean and Japanese designation simultaneously (e.g. 'East Sea/Sea of Japan') in your contents and maps.
According to IHO and UNCSGN, in case of topographical feature shared with two or more countries, yet naming differently in their own languages, all the names in each language should be marked.