Why are the signs in some airports and parks in China in addition to English, in Chinese, Japanese, and Korean? And I found that the same situation is true in Japan and South Korea.

English is the current international lingua franca, so this should be understandable.

Since South Korea, Japan, and China are East (South) Asian countries, these countries all have Chinese origins. Perhaps because of this close relationship, texts from Southeast Asian countries are often labeled together. In addition, Southeast Asian countries such as Vietnam and Myanmar also use Chinese characters.