Internationalizing Plugin

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You have to load text domain in your plugin:

<?php
function my_load_textdomain() {
    load_plugin_textdomain( 
        'add-to-post-footer', // text domain identifier
        false, // deprecated parameter
        plugin_basename(dirname( __FILE__ )) . '/languages' ); // path to the languages directory
}

add_action('plugins_loaded', 'my_load_textdomain');