Customize Permalinks for Each Language in WordPress

Code

<?php
// Add custom permalinks for different languages in WordPress

function wp_dudecom_custom_language_permalinks( $permalink, $post, $leavename ) {
    // Check if the post has a language set
    $language = get_post_meta( $post->ID, 'language', true );

    if ( $language ) {
        // Append the language code to the permalink
        $permalink = home_url( '/' . $language . '/' . $post->post_name . '/' );
    }

    return $permalink;
}
add_filter( 'post_link', 'wp_dudecom_custom_language_permalinks', 10, 3 );
add_filter( 'page_link', 'wp_dudecom_custom_language_permalinks', 10, 3 );

// Add rewrite rules for language-specific permalinks
function wp_dudecom_add_language_rewrite_rules() {
    $languages = array( 'en', 'fr', 'de' ); // Add your supported languages here

    foreach ( $languages as $language ) {
        add_rewrite_rule(
            '^' . $language . '/([^/]+)/?$',
            'index.php?name=$matches[1]&language=' . $language,
            'top'
        );
    }
}
add_action( 'init', 'wp_dudecom_add_language_rewrite_rules' );

// Add query vars for language
function wp_dudecom_add_language_query_vars( $vars ) {
    $vars[] = 'language';
    return $vars;
}
add_filter( 'query_vars', 'wp_dudecom_add_language_query_vars' );

// Modify the main query to filter by language
function wp_dudecom_filter_query_by_language( $query ) {
    if ( ! is_admin() && $query->is_main_query() && $language = get_query_var( 'language' ) ) {
        $meta_query = array(
            array(
                'key'     => 'language',
                'value'   => $language,
                'compare' => '='
            )
        );
        $query->set( 'meta_query', $meta_query );
    }
}
add_action( 'pre_get_posts', 'wp_dudecom_filter_query_by_language' );

// Ensure permalinks are flushed on activation
function wp_dudecom_flush_rewrite_rules() {
    wp_dudecom_add_language_rewrite_rules();
    flush_rewrite_rules();
}
register_activation_hook( __FILE__, 'wp_dudecom_flush_rewrite_rules' );

// Ensure permalinks are flushed on deactivation
function wp_dudecom_deactivate_plugin() {
    flush_rewrite_rules();
}
register_deactivation_hook( __FILE__, 'wp_dudecom_deactivate_plugin' );
?>