TestTranslation.php

<?php

namespace Drupal\media_test_source\Plugin\media\Source;

use Drupal\media\MediaInterface;

/**
 * Provides test media source.
 *
 * @MediaSource(
 *   id = "test_translation",
 *   label = @Translation("Test source with translations"),
 *   description = @Translation("Test media source with translations."),
 *   allowed_field_types = {"string"},
 *   thumbnail_alt_metadata_attribute = "test_thumbnail_alt",
 * )
 */
class TestTranslation extends Test {
    
    /**
     * {@inheritdoc}
     */
    public function getMetadata(MediaInterface $media, $attribute_name) {
        if ($attribute_name == 'thumbnail_uri') {
            return 'public://' . $media->language()
                ->getId() . '.png';
        }
        if ($attribute_name == 'test_thumbnail_alt') {
            $langcode = $media->language()
                ->getId();
            return $this->t('Test Thumbnail @language', [
                '@language' => $langcode,
            ], [
                'langcode' => $langcode,
            ]);
        }
        return parent::getMetadata($media, $attribute_name);
    }

}