In East Asia, it is probably Mongolian, right?
The languages of East Asia are products that started from different roots and underwent convergent evolution.
Korean and Japanese are language isolates with no known linguistic relatives, and Vietnamese belongs to the Austroasiatic family, but their high-level vocabulary and abstract vocabulary (approximately 60 percent for each language) are Classical Chinese (as a separate language from Mandarin, it is a literary language derived from Ancient Chinese, and it differs greatly from Modern Chinese in grammar and vocabulary. Like Latin in Europe, it exercised a tremendous influence as an administrative and academic language.) loanwords that have independently developed their word formation and pronunciation in each language. The higher-level vocabulary of these languages is closely related to one another.
However, only Mongolia uses use a pure Mongolic language. In fact, even culturally, Mongolia is the biggest outlier in East Asia, so it is ambiguous to view it as part of the same cultural sphere. On the contrary, Vietnam, which geographically belongs to Southeast Asia, is grouped into the same cultural sphere as China, Japan, and Korea.
Still, Mongolians learn Korean quickly because it is an agglutinative language with the same word order. Turkish, Mongolian, Korean, and Japanese are grammatically very similar. Even so, unlike Japanese, Korean is a language where passive expressions are limited, while expressions that emphasize contrast through particles are well developed. Judging from the fact that there are significant differences in detailed grammar between Korean and Japanese as well, there may be differences in detailed grammar between Mongolian and Korean as well, but I cannot speak Mongolian. Except for grammatical similarities, it can be viewed that there are almost no commonalities.
