open the file Customize.xml with Notepad in your custom pack folder on your computer then add
<allowed-language>zh</allowed-language>
<allowed-language>zs</allowed-language>
below
<allowed-language>en</allowed-language>
or above it, if you like it to display Chinese above English in the phone language list on your phone later. "zh" is Chinese Traditional, and "zs" is Chinese Simplified. Save the file then close it. Use XS++ to replace the file Customize.xml on your phone with the one edited on your computer.