mistvan

I figured this out.

I need to add loading text domain in your fuctions.php

add_action( 'plugins_loaded', 'wpsubscribe_load_textdomain' );
function wpsubscribe_load_textdomain() {
  load_plugin_textdomain( 'wp-subscribe', false, dirname( plugin_basename( __FILE__ ) ) . '/languages' );
}

And then in your Display widget section I use _e function with that text domain:
<?php _e($instance[‘success_message’], ‘wp-subscribe’); ?>.

Can you try to include that in your plugin for future updates please?