Set a Different Language for Individual Pages

$wpide_rtl_langs = array( 'ar', 'he', 'fa', 'ur', 'ps', 'yi', 'dv', 'ckb' ); // Codes written right-to-left.

add_action( 'init', function () {
    foreach ( array( 'post', 'page' ) as $type ) {
        register_post_meta( $type, '_wpide_lang', array(
            'type'              => 'string',
            'single'            => true,
            'show_in_rest'      => true,
            'sanitize_callback' => function ( $value ) {
                return preg_match( '/^[a-zA-Z]{2,3}(-[a-zA-Z0-9]{2,8})*$/', (string) $value ) ? $value : '';
            },
            'auth_callback'     => function ( $allowed, $key, $post_id ) {
                return current_user_can( 'edit_post', $post_id );
            },
        ) );
    }
} );

add_action( 'add_meta_boxes', function () {
    add_meta_box( 'wpide-lang', __( 'Page language' ), function ( $post ) {
        wp_nonce_field( 'wpide_lang_save', 'wpide_lang_nonce' );
        printf(
            '<label for="wpide_lang" class="screen-reader-text">%1$s</label><input type="text" id="wpide_lang" name="wpide_lang" value="%2$s" placeholder="%3$s" style="width:100%%"><p class="description">%4$s</p>',
            esc_html__( 'Language code' ),
            esc_attr( get_post_meta( $post->ID, '_wpide_lang', true ) ),
            esc_attr( str_replace( '_', '-', get_locale() ) ),
            esc_html__( 'Leave empty to use the site language.' )
        );
    }, array( 'post', 'page' ), 'side' );
} );

add_action( 'save_post', function ( $post_id ) {
    if ( ! isset( $_POST['wpide_lang_nonce'] ) || ! wp_verify_nonce( sanitize_key( $_POST['wpide_lang_nonce'] ), 'wpide_lang_save' ) || ! current_user_can( 'edit_post', $post_id ) ) {
        return;
    }

    $value = isset( $_POST['wpide_lang'] ) ? sanitize_text_field( wp_unslash( $_POST['wpide_lang'] ) ) : '';

    if ( '' === $value ) {
        delete_post_meta( $post_id, '_wpide_lang' );
    } else {
        update_post_meta( $post_id, '_wpide_lang', $value );
    }
} );

add_filter( 'language_attributes', function ( $output ) use ( $wpide_rtl_langs ) {
    if ( is_admin() || ! is_singular() ) {
        return $output;
    }

    $lang = get_post_meta( get_queried_object_id(), '_wpide_lang', true );

    if ( ! $lang ) {
        return $output;
    }

    $dir = in_array( strtolower( strtok( $lang, '-' ) ), $wpide_rtl_langs, true ) ? 'rtl' : 'ltr';

    return sprintf( 'dir="%1$s" lang="%2$s"', esc_attr( $dir ), esc_attr( $lang ) );
} );