For anyone who wants to localize this excelent plugin, there's a bug on line 826 that makes impossible localization/internationalization
Line 826 should be like this
load_plugin_textdomain('subscribe-to-comments','/wp-content/plugins/subscribe-to-comments/');
POT file is in extra folder, but localized PO files must be on subscribe-to-comments folder for this to work
I hope it will help to someone, or at least the author upgrades the plugin. Greetings.