First, follow the previous guide and install the both Simplified and Traditional Chinese languages files.
Then, press the Super/Windows key and type 'region', click on the menu item of 'Region & Language'. The 'Region & Language' window will pop up. Go the the 'Input Sources' section and click the '+' button.
Next, the system will prompt you to select the the languages you wan. Select the 'Chinese (China)' and you will be prompted again to select your input method. Select 'Chinese (SunPinyin)'. Once done, click the 'Add' button.
After you have added the 'Chinese (SunPinyin)' input source, the top panel will show the language selector. Click on the menu item and select 'zh'. If the menu item does not show up in top panel, log out and re-login again.
To switch between different input method, use 'Super/Windows + Space' keys to toggle it. Open up Gedit and start typing in Chinese as shown below.