LanguageTypes.php

<?php

namespace Drupal\language\Plugin\migrate\process;

use Drupal\migrate\MigrateException;
use Drupal\migrate\MigrateExecutableInterface;
use Drupal\migrate\ProcessPluginBase;
use Drupal\migrate\Row;

/**
 * Processes the array for the language types.
 *
 * @MigrateProcessPlugin(
 *   id = "language_types",
 *   handle_multiples = TRUE
 * )
 */
class LanguageTypes extends ProcessPluginBase {
    
    /**
     * {@inheritdoc}
     */
    public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
        if (!is_array($value)) {
            throw new MigrateException('The input should be an array');
        }
        if (array_key_exists('language', $value)) {
            $value['language_interface'] = $value['language'];
            unset($value['language']);
        }
        if (!empty($this->configuration['filter_configurable'])) {
            $value = array_filter($value);
        }
        return array_keys($value);
    }

}