It is an absolute mistake to hear just one side of the story and to blindly follow. If we leave these kinds of things alone, it causes serious problems that disturb the order of international society.
For your reference, Dorling & Kindersley, one of biggest textbook publishers, worldatlas.com, one of prominent online map provider, and one of the biggest mapmakers, National Geographic promised us that they would now use the name 'East Sea.' In addition, these websites are already using the name, 'East Sea' on their website after we pointed out the error.
According to IHO and UNCSGN, in case of topographical feature shared with two or more countries, yet naming differently in their own languages, all of the names in each language should be marked.
Once Korea and Japan agree on a common designation, that is in accord with the general rule of international cartography, we will then follow the agreed-on designation.