Redirect frontpage to the correct language

I wrote my own kernel request event subscriber for this that piggy backs off of the configuration form of the module:

namespace Drupal\my_module\EventSubscriber;

use Drupal\Core\Entity\EntityTypeManagerInterface;
use Drupal\Core\Path\PathMatcherInterface;
use Drupal\Core\Routing\TrustedRedirectResponse;
use Drupal\Core\Url;
use Drupal\language\Entity\ConfigurableLanguage;
use Drupal\language\LanguageNegotiatorInterface;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Symfony\Component\HttpKernel\Event\RequestEvent;
use Symfony\Component\HttpKernel\KernelEvents;

/**
 * Redirects to the relevant front page if no language code has been set.
 */
class IpCountryDetectionSubscriber implements EventSubscriberInterface {

  /**
   * Drupal\Core\Entity\EntityTypeManagerInterface definition.
   *
   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
   */
  protected EntityTypeManagerInterface $entityTypeManager;

  /**
   * The language negotiator.
   *
   * @var \Drupal\language\LanguageNegotiatorInterface
   */
  protected $negotiator;

  /**
   * Drupal\Core\Path\PathMatcherInterface definition.
   *
   * @var \Drupal\Core\Path\PathMatcherInterface
   */
  protected PathMatcherInterface $pathMatcher;

  /**
   * Constructs a IpCountryDetectionSubscriber.
   *
   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
   *   The entity type manager.
   * @param \Drupal\language\LanguageNegotiatorInterface $negotiator
   *   The language negotiator manager service.
   * @param \Drupal\Core\Path\PathMatcherInterface $path_matcher
   *   The path matcher service.
   */
  public function __construct(EntityTypeManagerInterface $entity_type_manager,
                              LanguageNegotiatorInterface $negotiator,
                              PathMatcherInterface $path_matcher) {
    $this->entityTypeManager = $entity_type_manager;
    $this->negotiator = $negotiator;
    $this->pathMatcher = $path_matcher;
  }

  /**
   * Redirects to the relevant front page if no language code has been set.
   *
   * The relevant front page is determined by ip2country and the
   * language negotiation subscriber from ip_language_negotiation.
   *
   * @param \Symfony\Component\HttpKernel\Event\RequestEvent $event
   *   The Event to process.
   */
  public function onKernelRequest(RequestEvent $event): void {
    $request = $event->getRequest();
    $path = $request->getPathInfo();

    // Only redirect if we have no language information in the URL.
    if ($path !== '/') {
      return;
    }

    try {
      $method_id = 'ip-language-negotiation-ip';
      $languageEntity = NULL;
      if ($this->negotiator->isNegotiationMethodEnabled($method_id)) {
        $ipLanguageNegotiation = $this->negotiator->getNegotiationMethodInstance($method_id);
        $langcode = $ipLanguageNegotiation->getLangcode($request);

        // Fallback is English (Europe) if IP detection fails.
        if ($langcode === NULL) {
          $langcode = 'en';
        }

        $languageEntity = $this->entityTypeManager->getStorage('configurable_language')
          ->load($langcode);
      }

      if ($languageEntity instanceof ConfigurableLanguage) {
        $request_query = $request->query->all();

        $route_name = $this->pathMatcher->isFrontPage() ? '<front>' : '<current>';
        $url = Url::fromRoute($route_name, [], ['language' => $languageEntity]);
        $url->setOption('query', (array) $url->getOption('query') + $request_query);

        $response = new TrustedRedirectResponse($url->setAbsolute()
          ->toString());
        $event->setResponse($response);
      }
    }
    catch (\Throwable $exception) {
    }
  }

  /**
   * {@inheritdoc}
   */
  public static function getSubscribedEvents() {
    $events = [];
    $events[KernelEvents::REQUEST][] = ['onKernelRequest', 400];
    return $events;
  }

}

Service definition:

my_module.kernel_request_subscriber:
    class: Drupal\my_module\EventSubscriber\IpCountryDetectionSubscriber
    arguments: [ '@entity_type.manager', '@language_negotiator', '@path.matcher' ]
    tags:
      - { name: event_subscriber }