/**
 * Implements hook_library_info_build().
 *
 * @return array
 */
function hook_library_info_build() {
  $libraries = [];

  /** @var \Drupal\Core\Language\LanguageManager $language_manager */
  $language_manager = \Drupal::languageManager();

  /** @var \Drupal\Core\Language\LanguageInterface[] $languages */
  $languages = $language_manager->getLanguages();

  foreach ($languages as $language) {
    $language_id = strtolower($language->getId());
    $libraries['mymodule.moment_with_locale_' . $language_id] = [
      'js' => [
        'js/vendor/moment/locale/' . $language_id . '.js' => [],
      ],
      'dependencies' => [
        'mymodule/moment',
      ],
    ];
  }

  return $libraries;
}

// attaching to build array
$build['#attached']['library'][] = 'mymodule/moment_with_locale_' . strtolower(\Drupal::languageManager()->getCurrentLanguage()->getId());