UrlAliasLanguage.php

<?php

namespace Drupal\path\Plugin\migrate\process\d6;

use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;
use Drupal\Core\Language\LanguageInterface;

/**
 * Url alias language code process.
 *
 * @MigrateProcessPlugin(
 *   id = "d6_url_alias_language"
 * )
 */
class UrlAliasLanguage extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        $langcode = $value === '' ? LanguageInterface::LANGCODE_NOT_SPECIFIED : $value;
        return $langcode;
    }

}