Question:
I’m trying to add a custom field to the “Menu Settings” section in the WordPress menu editor (/wp-admin/nav-menus.php). Specifically, I want to add a dropdown to select a language for each menu. This custom field should appear alongside the existing “Auto add pages” and “Display location” settings.

I’ve tried various hooks like wp_nav_menu, but I can’t get the custom field to appear in the correct location. Here is the area where I want to add the field (denoted by the red arrow in the image):

What I Want to Achieve:
Add a dropdown in the “Menu Settings” section.
Save the selected value when the menu is saved.
Retrieve and use the selected value in my theme.
Code I’ve Tried:

add_action('wp_nav_menu', 'add_language_field_to_menu_settings', 10, 2);

function add_language_field_to_menu_settings($nav_menu_selected_id) {
    $menu_id = $nav_menu_selected_id;
    $selected_language = get_option('menu_language_' . $menu_id, '');
    $available_languages = get_theme_available_languages();

    wp_nonce_field('save_menu_language_meta_box_data', 'menu_language_meta_box_nonce');

    echo '<div class="menu-settings">';
    echo '<h2>' . __('Menu Language') . '</h2>';
    echo '<label for="menu_language">' . __('Select Language') . '</label>';
    echo '<select name="menu_language" id="menu_language">';
    echo '<option value="" ' . selected($selected_language, '', false) . '>No Language Selected</option>';
    foreach ($available_languages as $lang_code => $first_line) {
        echo '<option value="' . esc_attr($lang_code) . '" ' . selected($selected_language, $lang_code, false) . '>' . esc_html($first_line) . '</option>';
    }
    echo '</select>';
    echo '</div>';
}

add_action('wp_update_nav_menu', 'save_menu_language_meta_box_data');

function save_menu_language_meta_box_data($menu_id) {
    if (!isset($_POST['menu_language_meta_box_nonce']) || !wp_verify_nonce($_POST['menu_language_meta_box_nonce'], 'save_menu_language_meta_box_data')) {
        return;
    }

    if ($menu_id) {
        $menu_language = isset($_POST['menu_language']) ? sanitize_text_field($_POST['menu_language']) : '';
        update_option('menu_language_' . $menu_id, $menu_language);
    }
}

function get_menu_language($menu_id) {
    return get_option('menu_language_' . $menu_id, '');
}

function get_theme_available_languages() {
    $languages = array();
    $language_dir = get_template_directory() . '/languages';

    if (is_dir($language_dir)) {
        $files = scandir($language_dir);
        foreach ($files as $file) {
            if ($file !== '.' && $file !== '..' && preg_match('/^[a-zA-Z_-]+.po$/', $file)) {
                $language_code = substr($file, 0, strpos($file, '.'));
                $file_path = $language_dir . '/' . $file;
                $first_line = '';
                $handle = fopen($file_path, 'r');
                if ($handle) {
                    $first_line = fgets($handle);
                    fclose($handle);
                }
                if (!empty($first_line)) {
                    $languages[$language_code] = trim($first_line);
                }
            }
        }
    }

    $languages['en-US'] = 'English (United States)'; // Ensure en-US is included
    return $languages;
}

What I Need Help With:

• Correctly adding the custom field to the “Menu Settings” section.
• Ensuring the selected value is saved and retrievable.

Any guidance or
solutions would be greatly appreciated!