TYPO3: Simply redirect a visitor to secondary language if browser does not support default language using PSR-15 Middleware

composer.json

{
  "name": "in2code/in2template",
  "description": "Sitepackage extension",
  "type": "typo3-cms-extension",
  "homepage": "https://www.in2code.de",
  "require": {
    "typo3/cms-core": "^10.4",
    "codezero/browser-locale": "^3.0"
  },
  "autoload": {
    "psr-4": {
      "In2code\\In2template\\": "Classes/"
    }
  }
}

LanguageRedirect.php

<?php
declare(strict_types=1);
namespace In2code\In2template\Middleware;

use CodeZero\BrowserLocale\BrowserLocale;
use CodeZero\BrowserLocale\Filters\LanguageFilter;
use In2code\In2template\Utility\ObjectUtility;
use Psr\Http\Message\ResponseInterface;
use Psr\Http\Message\ServerRequestInterface;
use Psr\Http\Server\MiddlewareInterface;
use Psr\Http\Server\RequestHandlerInterface;
use TYPO3\CMS\Core\Http\RedirectResponse;
use TYPO3\CMS\Core\Routing\PageArguments;
use TYPO3\CMS\Core\Site\Entity\Site;
use TYPO3\CMS\Core\Site\Entity\SiteLanguage;
use TYPO3\CMS\Core\Utility\GeneralUtility;
use TYPO3\CMS\Extbase\Object\Exception;

/**
 * Class LanguageRedirect
 * to redirect users to additional language of home site for browsers that do not support default language
 */
class LanguageRedirect implements MiddlewareInterface
{
    /**
     * @var ServerRequestInterface
     */
    protected $request = null;

    /**
     * @param ServerRequestInterface $request
     * @param RequestHandlerInterface $handler
     * @return ResponseInterface
     * @throws Exception
     */
    public function process(ServerRequestInterface $request, RequestHandlerInterface $handler): ResponseInterface
    {
        $this->request = $request;
        if ($this->isRedirectNeeded()) {
            return $this->redirectToSecondaryLanguage();
        }
        return $handler->handle($request);
    }

    /**
     * Redirect only if:
     * - Home page
     * - Referrer is not current domain (to enable a language switch on the website)
     * - If is default language
     * - Is no bot (do not redirect if a bot is watching your home page)
     * - Browser does not support default language
     * - Is additional language existing
     *
     * @return bool
     */
    protected function isRedirectNeeded(): bool
    {
        return $this->isHomepage() && $this->isExternalReferrer()
            && $this->isDefaultLanguage() && $this->isNoBot() && $this->isDefaultLanguageNotSupportedInVisitorBrowser()
            && $this->isAdditionalLanguageExisting();
    }

    /**
     * @return bool
     */
    protected function isNoBot(): bool
    {
        $bots = [
            'google',
            'bingbot',
            'slurp',
            'duckduck',
            'baidu',
            'yandex',
            'sogou',
            'exabot',
            'lighthouse'
        ];
        $userAgent = GeneralUtility::getIndpEnv('HTTP_USER_AGENT');
        if (empty($userAgent)) {
            // no useragent - crawler, caretaker or bot?
            return false;
        }
        foreach ($bots as $bot) {
            if (stristr($userAgent, $bot) !== false) {
                return false;
            }
        }
        return true;
    }

    /**
     * @return ResponseInterface
     * @throws Exception
     */
    protected function redirectToSecondaryLanguage(): ResponseInterface
    {
        $configuration = [
            'parameter' => $this->getHomePageIdentifier(),
            'language' => $this->getSecondaryLanguageIdentifier(),
            'forceAbsoluteUrl' => true
        ];
        $url = ObjectUtility::getContentObject()->typoLink_URL($configuration);
        return new RedirectResponse($url, 302);
    }

    /**
     * @return bool
     */
    protected function isHomepage(): bool
    {
        /** @var PageArguments $pageArguments */
        $pageArguments = $this->request->getAttribute('routing');
        return $pageArguments->getPageId() === $this->getHomePageIdentifier();
    }

    /**
     * Check if referrer is not within current domain (also empty referrer is used)
     *
     * @return bool
     */
    protected function isExternalReferrer(): bool
    {
        $referrer = GeneralUtility::getIndpEnv('HTTP_REFERER');
        $domain = GeneralUtility::getIndpEnv('HTTP_HOST');
        return stristr($referrer, $domain) === false;
    }

    /**
     * @return bool
     */
    protected function isDefaultLanguageNotSupportedInVisitorBrowser(): bool
    {
        try {
            $browser = new BrowserLocale(GeneralUtility::getIndpEnv('HTTP_ACCEPT_LANGUAGE'));
            $languages = $browser->filter(new LanguageFilter());
            return in_array($this->getDefaultLanguageCode(), $languages) === false;
        } catch (\Exception $exception) {
            return false;
        }
    }

    /**
     * @return bool
     */
    protected function isAdditionalLanguageExisting(): bool
    {
        $languages = $this->request->getAttribute('site')->getLanguages();
        return count($languages) > 1;
    }

    /**
     * @return bool
     */
    protected function isDefaultLanguage(): bool
    {
        /** @var SiteLanguage $language */
        $language = $this->request->getAttribute('language');
        return $language->getLanguageId() === $this->getDefaultLanguageIdentifier();
    }

    /**
     * Return language code (like "de") from default language configuration in site configuration
     *
     * @return string
     */
    protected function getDefaultLanguageCode(): string
    {
        /** @var SiteLanguage $defaultLanguage */
        $defaultLanguage = $this->request->getAttribute('site')->getDefaultLanguage();
        return $defaultLanguage->getTwoLetterIsoCode();
    }

    /**
     * @return int
     */
    protected function getDefaultLanguageIdentifier(): int
    {
        /** @var SiteLanguage $defaultLanguage */
        $defaultLanguage = $this->request->getAttribute('site')->getDefaultLanguage();
        return $defaultLanguage->getLanguageId();
    }

    /**
     * Get secondary language id from site configuration
     *
     * @return int
     */
    protected function getSecondaryLanguageIdentifier(): int
    {
        $languages = $this->request->getAttribute('site')->getLanguages();
        if (count($languages) > 1) {
            /** @var SiteLanguage $secondaryLanguage */
            $secondaryLanguage = $languages[1];
            return $secondaryLanguage->getLanguageId();
        }
        return 0;
    }

    /**
     * Get page id of the homepage from site configuration
     *
     * @return int
     */
    protected function getHomePageIdentifier(): int
    {
        /** @var Site $site */
        $site = $this->request->getAttribute('site');
        return $site->getRootPageId();
    }
}

RequestMiddlewares.php

<?php
return [
    'frontend' => [
        'in2template-languageredirect' => [
            'target' => \In2code\In2template\Middleware\LanguageRedirect::class,
            'after' => [
                'typo3/cms-frontend/tsfe',
                'typo3/cms-frontend/page-resolver',
            ]
        ]
    ]
];