If you want to display the translated day in your certificate then kindly follow the below step.
Open the file in this path /wp-content/plugins/designthemes-core-features/shortcodes/shortcodes.php using some editor like DreamWeaver and scroll to function dt_sc_certificate at line number : 2834 (Approx )
Here you can modify the code (
$out .= date('d M Y', $dt); ) like below,
$out .= date_i18n(get_option( 'date_format' ), $dt);
After that login to your Admin Panel and navigate to Settings > General there you need change the date format.