The title of your article has 'Japanisches Meer' in it.
But, as you know, this water is a heavily disputed area between South Korea and Japan.
Japan has been trying to claim this sea as its own against historical records.
But, what is for sure is East Sea has been the South Korean territory for so many years.
The name "the East Sea" appears in the History of the Three Kingdoms, one of the Korean ancient history books, in around 59B.C., while Japan's record of calling it "The Sea of Japan" appeared much later.
Not only that, in practice, the East Sea is under South Korea's control and the Dok islet in the East Sea is populated only by South Koreans.
The requirements for an islet to be a territory of a country are all fulfilled by South Korea, not Japan.
So, it is absolutely clear that the East Sea is legitimately South Korean water and therefore it is natural for it to be called the East Sea.
Even if we give Japan the benefit of doubt, international laws and regulations including that of the UN acknowledge that the East Sea is disputed and thus needs to be recognized as such.
UN recommends, "In case of a topographical feature under the sovereignty of more than one country if countries concerned failed to come to an agreement on a common name, it should be a general rule to accept all of the different names in each of the languages used by each of the countries concerned."