Assign Different Language Versions to WordPress Pages and Posts

Code

<?php
// Function to register a custom taxonomy for languages
function wp_dudecom_register_language_taxonomy() {
    $labels = array(
        'name'              => 'Languages',
        'singular_name'     => 'Language',
        'search_items'      => 'Search Languages',
        'all_items'         => 'All Languages',
        'edit_item'         => 'Edit Language',
        'update_item'       => 'Update Language',
        'add_new_item'      => 'Add New Language',
        'new_item_name'     => 'New Language Name',
        'menu_name'         => 'Languages',
    );

    $args = array(
        'hierarchical'      => true,
        'labels'            => $labels,
        'show_ui'           => true,
        'show_admin_column' => true,
        'query_var'         => true,
        'rewrite'           => array('slug' => 'language'),
    );

    register_taxonomy('language', array('post', 'page', 'custom_post_type'), $args);
}
add_action('init', 'wp_dudecom_register_language_taxonomy');

// Function to add a language switcher to the site
function wp_dudecom_language_switcher() {
    $languages = get_terms(array(
        'taxonomy'   => 'language',
        'hide_empty' => false,
    ));

    if (!empty($languages) && !is_wp_error($languages)) {
        echo '<ul class="language-switcher">';
        foreach ($languages as $language) {
            echo '<li><a href="' . esc_url(get_term_link($language)) . '">' . esc_html($language->name) . '</a></li>';
        }
        echo '</ul>';
    }
}

// Shortcode to display the language switcher
function wp_dudecom_language_switcher_shortcode() {
    ob_start();
    wp_dudecom_language_switcher();
    return ob_get_clean();
}
add_shortcode('language_switcher', 'wp_dudecom_language_switcher_shortcode');

// Function to filter posts by selected language
function wp_dudecom_filter_posts_by_language($query) {
    if (!is_admin() && $query->is_main_query() && (is_post_type_archive() || is_tax('language'))) {
        if (isset($_GET['language']) && !empty($_GET['language'])) {
            $query->set('tax_query', array(
                array(
                    'taxonomy' => 'language',
                    'field'    => 'slug',
                    'terms'    => sanitize_text_field($_GET['language']),
                ),
            ));
        }
    }
}
add_action('pre_get_posts', 'wp_dudecom_filter_posts_by_language');
?>