A sprachbund is a group of languages spoken in the same area. They become similar because the speakers live close to each other. They constantly communicate with each other. Because similarities in the languages develop, they sometimes seem to be in the same language family.
For example, in India, where there are hundreds or thousands of languages and dialects spoken, the languages often adopt similar grammar structures, vocabulary, and sounds. Indians often communicate with people who have a different first language from themselves.
In East Asia, since China introduced writing to nearby countries, especially Japan, Korea, and Vietnam. This area is called the sinosphere, or the Chinese sphere of influence. Not only did speakers of East Asian languages learn to by write using Chinese characters, but they also borrowed many loanwords from Chinese as well.
Linguists used to believe that languages like Japanese, Korean, Mongolian, Turkish, and Finnish were part of the Altaic language family. This was because of similar features like SVO (subject-verb-object) word order and agglutinating grammar. However, most linguists no longer believe that the Altaic language family ever existed. The languages became more similar over time, whereas most related languages become more different over time. It is possible that these languages are part of a sprachbund.