Source

File: wp-admin/includes/class-bulk-upgrader-skin.php

public function add_strings() {
		$this->upgrader->strings['skin_upgrade_start'] = __( 'The update process is starting. This process may take a while on some hosts, so please be patient.' );
		/* translators: 1: Title of an update, 2: Error message. */
		$this->upgrader->strings['skin_update_failed_error'] = __( 'An error occurred while updating %1$s: %2$s' );
		/* translators: %s: Title of an update. */
		$this->upgrader->strings['skin_update_failed'] = __( 'The update of %s failed.' );
		/* translators: %s: Title of an update. */
		$this->upgrader->strings['skin_update_successful'] = __( '%s updated successfully.' );
		$this->upgrader->strings['skin_upgrade_end']       = __( 'All updates have been completed.' );
	}