If you want to change default messages of the Registration Form, you can use any translation plugins or you can modify them using the available filter: pp_rf_custom_messages

To modify the messages using the filter, please add the following code to your current theme’s functions.php file:

add_filter( 'pp_rf_custom_messages', function( $messages ) {
	$messages['no_message'] = 'Registration successful!';
	$messages['on_fail'] = 'An error occurred. Please try again.';
	$messages['field_required'] = 'This field is required.';
	$messages['already_registered'] = 'You are already registered.';

	return $messages;
} );

After adding the code you just need to change the text.