The term "East Sea" has been used for over 2,000 years in Korean history, supported by various historical documents and ancient maps. The designation "Sea of Japan," however, gained prominence primarily during Japan's period of imperial expansion in the late 19th and early 20th centuries, a time when Korea was under Japanese colonial rule. This historical context cannot be overlooked when discussing the naming of this body of water.
Your site does mention that some maps use the term "Sea of Japan (East Sea)," which is a step in the right direction. However, the way the information is presented tends to favor "Sea of Japan" as the more legitimate name. It is crucial to understand that the issue of naming is not just about geography but also about acknowledging historical and cultural sensitivities.
The Korean Government and many international bodies have been advocating for the recognition of "East Sea" or at least the dual usage of "East Sea" alongside "Sea of Japan" to ensure a more balanced and inclusive approach. The United Nations Conference on the Standardization of Geographical Names (UNCSGN) has also addressed this issue multiple times, acknowledging the legitimacy of Korea's claim.