Which dialects became the foundation of the standard languages in Korea, China, and Japan?

- Standard Korean (South Korea):
The origin lies in the standard language established by the Korean Language Society in the 1930s. It was formulated based on the principle of "the speech of the middle class in Gyeongseong (Seoul)" and became the root of South Korean standard language after liberation.
- Chosun Cultural Language (North Korea):
Officially, it is defined as the "language of the proletarian class in Pyongyang," but in reality, it inherits the middle-class Seoul speech standards of the Korean Language Society while incorporating some Pyongan dialect. Therefore, Pyongyang speech and Cultural Language are distinguished; even Kim Jong-un speaks in the Pyongyang dialect to officials, but uses Cultural Language—which has intonation and vocabulary similar to Seoul speech—in official settings.
- Common Japanese:
In Edo (the former name of Tokyo), the dialect used by high-ranking samurai and their families in the daimyo villages of the western region became the standard. Based on this, the textbooks distributed in the 20th century and the intonation of NHK announcers developed into the standard language. Conversely, "Edoben," spoken by the native Edo residents in the commoner neighborhoods of eastern Tokyo, is still considered a dialect today.
- Standard Chinese (China, Taiwan, Singapore, and Chinese-speaking regions worldwide):
The starting point was "Guanhua," created based on Northern dialects to facilitate communication among high-ranking officials who moved from various regions to Beijing during the Ming and Qing dynasties. During the Qing dynasty, Classical Chinese became the official written language, while Guanhua became the official spoken language, which the West called Mandarin. Later, in the 20th century, Guanhua was developed into Standard Chinese. This is why it differs from the Beijing dialect.
Among the dialects that became the foundation of standard languages, the one where the boundary with the standard language has become blurred is Seoul speech... since the characteristics of the Seoul dialect have been rapidly disappearing since the 2000s.
Although Xi Jinping is from Beijing, he is famous for his perfect standard language speeches with almost no Beijing dialect. Because he lived from a young age in an area for high-ranking officials from all over the country located in central Beijing, he grew up in a linguistic environment of standard language with almost no characteristics of the Beijing dialect.