Being geographically close to each other, it is not surprising that China, Korea and Japan share similarities not only in martial arts, but also in culture and language. For example, the Chinese Long Saber manual 單刀法選 (dan dao fa xuan) was written based on combining Chinese Shaolin Staff and Japanese sword techniques.
On the right, you can see 2 examples of a Japanese manual and a Korean manual, with pages written entirely in Chinese. If you understood Chinese, you'll be able to read them.
To gain deeper and richer insights into the history of Eastern martial arts, it is important that we triangulate our information from these 3 cultures.