1. Upload lang/chinese-cn.php to your whmcs_install_dir/lang/ path.
if you current use WHMCS v6.3.0 or higher, you should rename original chinese.php to chinese-tw.php manually. Because original chinese.php was a Traditional Chinese edition.
2. Upload admin/lang/chinese-cn.php to your whmcs_install_dir/admin_dir/lang/ path.
1. Click the "My Account", Change Language to "Chinese-cn", enter your admin password at the last "Confirm Password" line, after click the "Save Changes" button save your changes.
2. Goto "System Settings > General Settings", enter your admin passowrd and click the "Confirm Password" button, then switch to "Localisation" tab, set "Default Language" to "Chinese-cn", at last click the "Save Changes" button save your changes.
If you want to help me improve chinese translations, please fork and send pull request to me.