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. Sign out your admin account, then choose Chinese language on login page.
2. Click the "My Account", Change language to Chinese, Enter your admin password at "Confirm Password" line, at last click "Save Changes" button.
3. Goto "Setup > General Settings", click "Localisation" tab, set "Default Language" to Chinese, at last click "Save Changes" button.
If you want to help me improve chinese translations, please fork and send pull request to me.