If you simply take a fresh install of Ubuntu and try to enable Chinese input, it won't do anything. The solution is here.
First, install the package ibus-sunpinyin (you can try ibus-pinyin but I found it annoying to use):
sudo apt install ibus-sunpinyin
ibus restart
Then go to System Settings -> Language Support and install Chinese. You need to logout and login for the changes to take effect.
Finally, in System Settings -> Text Entry, you should be able to add Chinese (SunPinyin) (IBus).
Test characters: 系，将，外，深，腿，浅，亲
Now you may notice some characters being rendered strangely (by default they may be rendered as Japanese rather than Chinese characters). This post outlines the issue and how to solve it.