'East sea' was used for 2000 years but due to Japanese occupation the sea started to be called as 'Sea of Japan'. However because the sea between Korea and Japan is Korea's territory it should be called as 'East Sea'. On the historical record of Korea, Japan, and China, it is easy to find that that part of the sea is written as 'East Sea'.
Also, IHO(International Hydrographic Organization) chose 'East Sea' as an official name of that sea. For these reasons 'East Sea' is the international standard name.