In order to search files, I want to input Chinese. But when I point to the input field. It shows English keyboard and I cannot change the language input. Why? Thank you very much.
When preferences are set in iOS through Settings.app, they apply to the current user, which is mobile. These preferences are written to ~/Library/Preferences.
Apps such as iFile which run as root obtain their preferences from the root user, which by factory is set the same on all devices. Changing settings in mobile doesn't affect root, as it should. This is useful in case you make a change that means the mobile user cannot log in.
This is also reflected in that Cydia Substrate which runs as mobile is not in effect in root apps such as iFile. That Cydia runs as mobile now has lead to a number of Substrate tweaks which were not possible before.
I'm not certain how to change root preferences, but I wouldn't want to either as changes to root could break its ability to log in which may cause iOS to not be able to log in. I recommend you copy text from another app which runs as mobile and allows you to use your preferences in keyboard.