Multilingual sites using alternate form of URL language negotiation are treated as monolingual

This does not work for me as I have "Customize Content language detection to differ from Interface text language detection settings" unchecked under admin/config/regional/language/detection.

I don't really grasp what LanguageInterface::TYPE_URL does. If it does what I think it does, this should be a working solution:

  public static function isMultilingualSitemap() {

    /** @var \Drupal\language\LanguageNegotiatorInterface $language_negotiator */
    $language_negotiator = \Drupal::service('language_negotiator');

    $url_negotiation_method_enabled = FALSE;
    foreach ($language_negotiator->getNegotiationMethods(LanguageInterface::TYPE_URL) as $method) {
      if ($language_negotiator->isNegotiationMethodEnabled($method['id'])) {
        $url_negotiation_method_enabled = TRUE;
        break;
      }
    }

    $has_multiple_indexable_languages = count(
        array_diff_key(\Drupal::languageManager()->getLanguages(),
          \Drupal::service('simple_sitemap.generator')->getSetting('excluded_languages', []))
      ) > 1;

    return $url_negotiation_method_enabled && $has_multiple_indexable_languages;
  }

Otherwise this might be necessary:

  public static function isMultilingualSitemap() {

    /** @var \Drupal\language\LanguageNegotiatorInterface $language_negotiator */
    $language_negotiator = \Drupal::service('language_negotiator');

    $url_negotiation_method_enabled = FALSE;
    foreach ([LanguageInterface::TYPE_CONTENT, LanguageInterface::TYPE_INTERFACE] as $type) {
      foreach ($language_negotiator->getNegotiationMethods($type) as $method) {
        if ($language_negotiator->getNegotiationMethodInstance($method['id']) instanceof LanguageNegotiationUrl
          && $language_negotiator->isNegotiationMethodEnabled($method['id'])) {
          $url_negotiation_method_enabled = TRUE;
          break 2;
        }
      }
    }

    $has_multiple_indexable_languages = count(
        array_diff_key(\Drupal::languageManager()->getLanguages(),
          \Drupal::service('simple_sitemap.generator')->getSetting('excluded_languages', []))
      ) > 1;

    return $url_negotiation_method_enabled && $has_multiple_indexable_languages;
  }

Both are working for me. Sorry for not creating patches, but I am quite busy right now. Could you please review/test these two approaches and provide feedback?

Any progress on this?

Marking this as major. One of the live site we are working on doesn't has URL detection enabled. So its not creating for the links with the lang-code.

@abhisekmazumdar Please correct me if I am wrong, but a multilingual sitemap is only practical/functional if it can take you to a specific translation/language. If URL detection is fully disabled, the links in the sitemap will be same for each language. What is the point of having repeating links for each language?

Thanks for testing.

As far as I understand, this issue hasn't been solved because it still requires the default URL language detection to be enabled.
I don't consider reopening this issue, because I consider #3228584: Multilingual sitemap test fails for some language negotiation plugins to be a follow-up issue.