functions.php

<?php
require __DIR__ . '/classes/HMT_Language_Switcher_Widget.php';

// ...

/**
 * Register our sidebars and widgetized areas.
 */
function hmt_widgets_init()
{
    register_sidebar(array(
        'name' => __('Trailing Main Navigation'),
        'id' => 'trailing_main_nav',
        'before_widget' => '<div>',
        'after_widget' => '</div>',
        'before_title' => '<h2>',
        'after_title' => '</h2>',
    ));
}
add_action('widgets_init', 'hmt_widgets_init');

/**
 * Register our widgets.
 */
function hmt_language_switcher_widget()
{
    register_widget('HMT_Language_Switcher_Widget');
}
add_action('widgets_init', 'hmt_language_switcher_widget');