diff --git a/typo3/sysext/backend/Classes/Controller/SiteConfigurationController.php b/typo3/sysext/backend/Classes/Controller/SiteConfigurationController.php
index c5c3b1956e769aa9f04ff78731ad1c8b6a005709..175c6439992ba9307df8de0541027a498fe5423e 100644
--- a/typo3/sysext/backend/Classes/Controller/SiteConfigurationController.php
+++ b/typo3/sysext/backend/Classes/Controller/SiteConfigurationController.php
@@ -825,6 +825,9 @@ class SiteConfigurationController
             if (isset($languageConfiguration['typo3Language'])) {
                 $existingLanguagesWithLegacyProperties[$languageConfiguration['languageId']]['typo3Language'] = $languageConfiguration['typo3Language'];
             }
+            if (isset($languageConfiguration['iso-639-1'])) {
+                $existingLanguagesWithLegacyProperties[$languageConfiguration['languageId']]['iso-639-1'] = $languageConfiguration['iso-639-1'];
+            }
         }
         foreach ($newSysSiteData['languages'] ?? [] as $key => $languageConfiguration) {
             $languageId = $languageConfiguration['languageId'];
diff --git a/typo3/sysext/backend/Classes/Controller/SiteInlineAjaxController.php b/typo3/sysext/backend/Classes/Controller/SiteInlineAjaxController.php
index bab5cccc5029723f4e4c855fc11dc905a19e4e27..b09a5a0c38db339f5fe4d7cf1e937acc322eeaad 100644
--- a/typo3/sysext/backend/Classes/Controller/SiteInlineAjaxController.php
+++ b/typo3/sysext/backend/Classes/Controller/SiteInlineAjaxController.php
@@ -92,11 +92,8 @@ class SiteInlineAjaxController extends AbstractFormEngineAjaxController
                     if ($language->getTitle() !== '') {
                         $defaultDatabaseRow['title'] = $language->getTitle();
                     }
-                    if ($language->getTwoLetterIsoCode() !== '') {
-                        $defaultDatabaseRow['iso-639-1'] = $language->getTwoLetterIsoCode();
-                        if ($language->getBase()->getPath() !== '/') {
-                            $defaultDatabaseRow['base'] = '/' . $language->getTwoLetterIsoCode() . '/';
-                        }
+                    if ($language->getBase()->getPath() !== '/') {
+                        $defaultDatabaseRow['base'] = '/' . strtolower($language->getLocale()->getName()) . '/';
                     }
                     if ($language->getNavigationTitle() !== '') {
                         $defaultDatabaseRow['navigationTitle'] = $language->getNavigationTitle();
diff --git a/typo3/sysext/backend/Classes/Form/FormDataProvider/DatabaseSystemLanguageRows.php b/typo3/sysext/backend/Classes/Form/FormDataProvider/DatabaseSystemLanguageRows.php
index 18b9b91928202e8c1270f2398603ad9b3a5dce78..3044b0873119382352c1746e897caaa7a4756fdc 100644
--- a/typo3/sysext/backend/Classes/Form/FormDataProvider/DatabaseSystemLanguageRows.php
+++ b/typo3/sysext/backend/Classes/Form/FormDataProvider/DatabaseSystemLanguageRows.php
@@ -51,7 +51,7 @@ class DatabaseSystemLanguageRows implements FormDataProviderInterface
         foreach ($languages as $language) {
             $languageId = $language->getLanguageId();
             if ($languageId > 0) {
-                $iso = $language->getTwoLetterIsoCode();
+                $iso = $language->getLocale()->getLanguageCode();
             } else {
                 $iso = 'DEF';
             }
@@ -74,7 +74,7 @@ class DatabaseSystemLanguageRows implements FormDataProviderInterface
                 // @todo: since the rest of the FormEngine code does not rely on iso code?
                 $message = sprintf(
                     $this->getLanguageService()->sL('LLL:EXT:core/Resources/Private/Language/locallang_core.xlf:error.missingLanguageIsocode'),
-                    $language->getTwoLetterIsoCode(),
+                    $language->getLocale()->getLanguageCode(),
                     $languageId
                 );
                 $flashMessage = GeneralUtility::makeInstance(
diff --git a/typo3/sysext/backend/Classes/Form/FormDataProvider/SiteTcaSelectItems.php b/typo3/sysext/backend/Classes/Form/FormDataProvider/SiteTcaSelectItems.php
deleted file mode 100644
index 053c712e71e3e66b6481552eeefe6318f85ea0c6..0000000000000000000000000000000000000000
--- a/typo3/sysext/backend/Classes/Form/FormDataProvider/SiteTcaSelectItems.php
+++ /dev/null
@@ -1,50 +0,0 @@
-<?php
-
-declare(strict_types=1);
-
-/*
- * This file is part of the TYPO3 CMS project.
- *
- * It is free software; you can redistribute it and/or modify it under
- * the terms of the GNU General Public License, either version 2
- * of the License, or any later version.
- *
- * For the full copyright and license information, please read the
- * LICENSE.txt file that was distributed with this source code.
- *
- * The TYPO3 project - inspiring people to share!
- */
-
-namespace TYPO3\CMS\Backend\Form\FormDataProvider;
-
-use TYPO3\CMS\Backend\Form\FormDataProviderInterface;
-use TYPO3\CMS\Core\Service\IsoCodeService;
-use TYPO3\CMS\Core\Utility\GeneralUtility;
-
-/**
- * Special data provider for the sites configuration module.
- *
- * Resolve some specialities of the "site configuration"
- */
-class SiteTcaSelectItems implements FormDataProviderInterface
-{
-    /**
-     * Resolve select items for
-     * * 'site_language' -> 'typo3language'
-     *
-     * @throws \UnexpectedValueException
-     */
-    public function addData(array $result): array
-    {
-        $table = $result['tableName'];
-        if ($table !== 'site_language') {
-            return $result;
-        }
-
-        // Available ISO-639-1 codes fetch from service class and put as "iso-639-1" items
-        $isoItems = GeneralUtility::makeInstance(IsoCodeService::class)->renderIsoCodeSelectDropdown(['items' => []]);
-        $result['processedTca']['columns']['iso-639-1']['config']['items'] = $isoItems['items'];
-
-        return $result;
-    }
-}
diff --git a/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaSiteLanguage.php b/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaSiteLanguage.php
index 3d0405ef05a958dbf958e6969f042dc84b1229f8..db5ec3a6c2970053f562294ecc3211c2ecd02a23 100644
--- a/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaSiteLanguage.php
+++ b/typo3/sysext/backend/Classes/Form/FormDataProvider/TcaSiteLanguage.php
@@ -303,9 +303,6 @@ class TcaSiteLanguage extends AbstractDatabaseRecordProvider implements FormData
                     if ($language->getTitle() !== '') {
                         $defaultDatabaseRow['title'] = $language->getTitle();
                     }
-                    if ($language->getTwoLetterIsoCode() !== '') {
-                        $defaultDatabaseRow['iso-639-1'] = $language->getTwoLetterIsoCode();
-                    }
                     if ($language->getNavigationTitle() !== '') {
                         $defaultDatabaseRow['navigationTitle'] = $language->getNavigationTitle();
                     }
diff --git a/typo3/sysext/backend/Configuration/SiteConfiguration/site_language.php b/typo3/sysext/backend/Configuration/SiteConfiguration/site_language.php
index 7dd89a81febdb0f7abcad095fb4a5ba7ea5c46b8..0ae47acc93a9d87cf7cf7b6ae7b4f8e02f3f4bfc 100644
--- a/typo3/sysext/backend/Configuration/SiteConfiguration/site_language.php
+++ b/typo3/sysext/backend/Configuration/SiteConfiguration/site_language.php
@@ -66,23 +66,13 @@ return [
                 'required' => true,
                 'eval' => 'trim',
                 'size' => 20,
-                'placeholder' => 'en_US.UTF-8',
+                'placeholder' => 'en-US',
                 'valuePicker' => [
                     'mode' => '',
                     'items' =>\TYPO3\CMS\Backend\Configuration\TCA\UserFunctions::getAllSystemLocales(),
                 ],
             ],
         ],
-        'iso-639-1' => [
-            'label' => 'LLL:EXT:backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf:site_language.iso-639-1',
-            'description' => 'LLL:EXT:backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf:site_language.iso-639-1',
-            'config' => [
-                'type' => 'select',
-                'renderType' => 'selectSingle',
-                // Fed by data provider
-                'items' => [],
-            ],
-        ],
         'hreflang' => [
             'label' => 'LLL:EXT:backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf:site_language.hreflang',
             'description' => 'LLL:EXT:backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf:site_language.hreflang',
@@ -427,7 +417,7 @@ return [
     ],
     'types' => [
         '1' => [
-            'showitem' => '--palette--;;default, --palette--;;locale-related, --palette--;;rendering-related, flag, --palette--;;languageIdPalette',
+            'showitem' => '--palette--;;default, locale, --palette--;;rendering-related, flag, --palette--;;languageIdPalette',
         ],
     ],
     'palettes' => [
@@ -438,10 +428,6 @@ return [
             'showitem' => 'languageId',
             'isHiddenPalette' => true,
         ],
-        'locale-related' => [
-            'label' => 'LLL:EXT:backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf:site_language.palette.locales',
-            'showitem' => 'locale, iso-639-1',
-        ],
         'rendering-related' => [
             'label' => 'LLL:EXT:backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf:site_language.palette.frontend',
             'showitem' => 'websiteTitle, --linebreak--, navigationTitle, hreflang, direction, --linebreak--, fallbackType, --linebreak--, fallbacks',
diff --git a/typo3/sysext/backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf b/typo3/sysext/backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf
index 8d3cf23caa6a7878359a85185577901e1a56e590..bb85849144bcddf78a6c2539ec578aa772a55fe9 100644
--- a/typo3/sysext/backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf
+++ b/typo3/sysext/backend/Resources/Private/Language/locallang_siteconfiguration_tca.xlf
@@ -51,10 +51,6 @@
 			<trans-unit id="site.tab.routes" resname="site.tab.routes">
 				<source>Static Routes</source>
 			</trans-unit>
-
-			<trans-unit id="site_language.palette.locales" resname="site_language.palette.locales">
-				<source>Locales</source>
-			</trans-unit>
 			<trans-unit id="site_language.palette.frontend" resname="site_language.palette.frontend">
 				<source>Frontend related</source>
 			</trans-unit>
@@ -82,9 +78,6 @@
 			<trans-unit id="site_language.locale" resname="site_language.locale">
 				<source>Locale</source>
 			</trans-unit>
-			<trans-unit id="site_language.iso-639-1" resname="site_language.iso-639-1">
-				<source>Two Letter ISO Code</source>
-			</trans-unit>
 			<trans-unit id="site_language.hreflang" resname="site_language.hreflang">
 				<source>Language Tag defined by RFC 1766 / 3066</source>
 			</trans-unit>
diff --git a/typo3/sysext/backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf b/typo3/sysext/backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf
index c9d8815e4691b7b62e6ecadd944ba3a116346cca..90f9997c1738ae11563af28ac9da6b0947ea2a73 100644
--- a/typo3/sysext/backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf
+++ b/typo3/sysext/backend/Resources/Private/Language/siteconfiguration_fieldinformation.xlf
@@ -24,9 +24,6 @@
 			<trans-unit id="site_language.locale" resname="site_language.locale">
 				<source>Used for localized date and currency formats. E.g. "de_DE" or "en_US.UTF-8".</source>
 			</trans-unit>
-			<trans-unit id="site_language.iso-639-1" resname="site_language.iso-639-1">
-				<source>ISO 639-1 code of the Language</source>
-			</trans-unit>
 			<trans-unit id="site_language.hreflang" resname="site_language.hreflang">
 				<source>Used within for "lang" and "hreflang" attributes</source>
 			</trans-unit>
diff --git a/typo3/sysext/backend/Tests/Functional/Form/FormDataProvider/TcaSelectItemsTest.php b/typo3/sysext/backend/Tests/Functional/Form/FormDataProvider/TcaSelectItemsTest.php
index 2e17d76cfa78cbe730a9e6e107097faaffbfec24..06161d6ef35228f7a74e078821a8210b002c1441 100644
--- a/typo3/sysext/backend/Tests/Functional/Form/FormDataProvider/TcaSelectItemsTest.php
+++ b/typo3/sysext/backend/Tests/Functional/Form/FormDataProvider/TcaSelectItemsTest.php
@@ -1754,19 +1754,16 @@ class TcaSelectItemsTest extends FunctionalTestCase
                             'languageId' => 0,
                             'title' => 'Default',
                             'locale' => 'en_US.UTF-8',
-                            'iso-639-1' => 'en',
                         ],
                         [
                             'languageId' => 1,
                             'title' => 'German',
                             'locale' => 'de_DE.UTF-8',
-                            'iso-639-1' => 'de',
                         ],
                         [
                             'languageId' => 2,
                             'title' => 'Danish',
                             'locale' => 'da_DK.UTF-8',
-                            'iso-639-1' => 'da',
                         ],
                     ],
                 ]
diff --git a/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/DatabaseSystemLanguageRowsTest.php b/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/DatabaseSystemLanguageRowsTest.php
index 80fe6e2eb0f38026fd8cdcb34858e153496364fa..6b03b9f98eb9e549e05ea5b865c7d7f5ee2df3ea 100644
--- a/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/DatabaseSystemLanguageRowsTest.php
+++ b/typo3/sysext/backend/Tests/Unit/Form/FormDataProvider/DatabaseSystemLanguageRowsTest.php
@@ -55,15 +55,15 @@ class DatabaseSystemLanguageRowsTest extends UnitTestCase
         );
         $siteLanguageZero = new SiteLanguage(
             0,
-            '',
+            'en',
             new Uri('/en/'),
             ['title' => 'English', 'flag' => 'empty-empty', 'direction' => 'ltr']
         );
         $siteLanguageOne = new SiteLanguage(
             1,
-            '',
+            'nl_NL',
             new Uri('/nl/'),
-            ['title' => 'Dutch', 'flag' => 'flag-nl', 'direction' => 'rtl', 'iso-639-1' => 'NL']
+            ['title' => 'Dutch', 'flag' => 'flag-nl', 'direction' => 'rtl']
         );
         $siteLanguages = [
             $siteLanguageMinusOne,
@@ -95,7 +95,7 @@ class DatabaseSystemLanguageRowsTest extends UnitTestCase
                 1 => [
                     'uid' => 1,
                     'title' => 'Dutch',
-                    'iso' => 'NL',
+                    'iso' => 'nl',
                     'flagIconIdentifier' => 'flag-nl',
                     'direction' => 'rtl',
                 ],
diff --git a/typo3/sysext/core/Classes/Configuration/SiteConfiguration.php b/typo3/sysext/core/Classes/Configuration/SiteConfiguration.php
index 0dff175927b1be3ec6949eaf433352ed75c40484..0bfc63c082361a396961a1356810e2f81ea091ab 100644
--- a/typo3/sysext/core/Classes/Configuration/SiteConfiguration.php
+++ b/typo3/sysext/core/Classes/Configuration/SiteConfiguration.php
@@ -128,7 +128,6 @@ class SiteConfiguration implements SingletonInterface
                     'languageId' => 0,
                     'base' => '/',
                     'locale' => 'en_US.UTF-8',
-                    'iso-639-1' => 'en',
                     'navigationTitle' => 'English',
                     'hreflang' => 'en-us',
                     'direction' => 'ltr',
diff --git a/typo3/sysext/core/Classes/Site/Entity/Site.php b/typo3/sysext/core/Classes/Site/Entity/Site.php
index 1fd3ab58e33b3882a91d964946766d91d33b4394..ade7128b2562531bff9d8c88a8c55266f55beb92 100644
--- a/typo3/sysext/core/Classes/Site/Entity/Site.php
+++ b/typo3/sysext/core/Classes/Site/Entity/Site.php
@@ -98,7 +98,6 @@ class Site implements SiteInterface
                 'navigationTitle' => '',
                 'flag' => 'us',
                 'locale' => 'en_US.UTF-8',
-                'iso-639-1' => 'en',
                 'hreflang' => 'en-US',
                 'direction' => '',
             ],
diff --git a/typo3/sysext/core/Classes/Site/Entity/SiteLanguage.php b/typo3/sysext/core/Classes/Site/Entity/SiteLanguage.php
index 352b03f7feb77871aff1970c2d4e9c7111494cbd..17455e6bd32f4eaeffc186cf2ac608a7c13c7bff 100644
--- a/typo3/sysext/core/Classes/Site/Entity/SiteLanguage.php
+++ b/typo3/sysext/core/Classes/Site/Entity/SiteLanguage.php
@@ -70,9 +70,10 @@ class SiteLanguage
 
     /**
      * The iso code for this language (two letter) ISO-639-1
+     * @deprecated in favor of $this->locale
      * @var string
      */
-    protected $twoLetterIsoCode = 'en';
+    protected $twoLetterIsoCode = '';
 
     /**
      * Language tag for this language defined by RFC 1766 / 3066 for "lang"
@@ -188,7 +189,8 @@ class SiteLanguage
             'title' => $this->getTitle(),
             'websiteTitle' => $this->getWebsiteTitle(),
             'navigationTitle' => $this->getNavigationTitle(),
-            'twoLetterIsoCode' => $this->getTwoLetterIsoCode(),
+            // @deprecated will be removed in TYPO3 v13.0
+            'twoLetterIsoCode' => $this->twoLetterIsoCode ?: $this->locale->getLanguageCode(),
             'hreflang' => $this->getHreflang(),
             'direction' => $this->getDirection(),
             'typo3Language' => $this->getTypo3Language(),
@@ -258,10 +260,12 @@ class SiteLanguage
 
     /**
      * Returns the ISO-639-1 language ISO code
+     * @deprecated not needed anymore, use $this->getLocale()->getLanguageCode() instead.
      */
     public function getTwoLetterIsoCode(): string
     {
-        return $this->twoLetterIsoCode;
+        trigger_error('SiteLanguage->getTwoLetterIsoCode() will be removed in TYPO3 v13.0. User SiteLanguage->getLocale()->getLanguageCode() instead.', E_USER_DEPRECATED);
+        return $this->twoLetterIsoCode ?: $this->locale->getLanguageCode();
     }
 
     /**
diff --git a/typo3/sysext/core/Configuration/DefaultConfiguration.php b/typo3/sysext/core/Configuration/DefaultConfiguration.php
index a89e7c9cef60f8c2de35ab8f918e962758576a19..c8f7ca985ec371ed28a14bf43ac799fe45d358ba 100644
--- a/typo3/sysext/core/Configuration/DefaultConfiguration.php
+++ b/typo3/sysext/core/Configuration/DefaultConfiguration.php
@@ -1172,11 +1172,6 @@ return [
                             \TYPO3\CMS\Backend\Form\FormDataProvider\TcaGroup::class,
                         ],
                     ],
-                    \TYPO3\CMS\Backend\Form\FormDataProvider\SiteTcaSelectItems::class => [
-                        'depends' => [
-                            \TYPO3\CMS\Backend\Form\FormDataProvider\TcaFolder::class,
-                        ],
-                    ],
                     \TYPO3\CMS\Backend\Form\FormDataProvider\TcaSelectItems::class => [
                         'depends' => [
                             \TYPO3\CMS\Backend\Form\FormDataProvider\DatabasePageRootline::class,
@@ -1185,7 +1180,6 @@ return [
                             \TYPO3\CMS\Backend\Form\FormDataProvider\TcaTypesShowitem::class,
                             \TYPO3\CMS\Backend\Form\FormDataProvider\TcaColumnsRemoveUnused::class,
                             \TYPO3\CMS\Backend\Form\FormDataProvider\TcaCheckboxItems::class,
-                            \TYPO3\CMS\Backend\Form\FormDataProvider\SiteTcaSelectItems::class,
                         ],
                     ],
                     \TYPO3\CMS\Backend\Form\FormDataProvider\TcaInlineConfiguration::class => [
diff --git a/typo3/sysext/core/Documentation/Changelog/12.3/Deprecation-99905-SiteLanguageIso-639-1Setting.rst b/typo3/sysext/core/Documentation/Changelog/12.3/Deprecation-99905-SiteLanguageIso-639-1Setting.rst
new file mode 100644
index 0000000000000000000000000000000000000000..b3fecff592539e63161db7c1736ced70de4a91d0
--- /dev/null
+++ b/typo3/sysext/core/Documentation/Changelog/12.3/Deprecation-99905-SiteLanguageIso-639-1Setting.rst
@@ -0,0 +1,67 @@
+.. include:: /Includes.rst.txt
+
+.. _deprecation-99905-1675963182:
+
+=======================================================
+Deprecation: #99905 - Site Language "iso-639-1" setting
+=======================================================
+
+See :issue:`99905`
+
+Description
+===========
+
+A language configuration defined for a site has had various settings, one of
+them being "iso-639-1" (also known as "twoLetterIsoCode").
+
+This setting was previously introduced to define the actual ISO 639-1 code, which
+was different from the Locale or the "typo3Language" setting. However,
+this information can now properly retrieved from the method:
+:php:`SiteLanguage->getLocale()->getLanguageCode()`.
+
+Since TYPO3 v12 it is not needed to set this property in the site configuration
+anymore, and it is removed from the Backend UI. The information is now automatically
+derived from the Locale setting of the Site Configuration.
+
+This property originally came from an option in TypoScript called
+:typoscript:`config.sys_language_isocode` which in turn was created in favor of
+the previous `sys_language` database table. TYPO3 Core never evaluated this
+setting properly before TYPO3 v9.
+
+As a result, the amount of options in the User Interface for integrators is
+reduced.
+
+The PHP method :php:`SiteLanguage->getTwoLetterIsoCode()` serves now purpose
+anymore and is deprecated.
+
+This also affects the TypoScript "getData" property :typoscript:`siteLanguage:twoLetterIsoCode`,
+and the TypoScript condition :typoscript:`[siteLanguage("twoLetterIsoCode")]`.
+
+
+Impact
+======
+
+Using the TypoScript settings or the PHP method will trigger a PHP deprecation notice.
+
+An Administrator can not select a value for the "iso-639-1" setting anymore
+via the TYPO3 Backend. However, when saving a site configuration via the
+TYPO3 Backend will still keep the "iso-639-1" setting so no information is lost.
+
+
+Affected installations
+======================
+
+TYPO3 installation actively accessing this property via PHP or TypoScript.
+
+
+Migration
+=========
+
+No migration is needed as the explicit option is still evaluated. It is however
+recommended to evaluate if the setting is really needed, and if the first part of the
+`locale` setting matches the `iso-639-1` setting. If so, the line with `iso-639-1` can be removed.
+
+As for TypoScript, it is recommended to use :typoscript:`siteLanguage:locale:languageCode`
+instead of :typoscript:`siteLanguage:twoLetterIsoCode`.
+
+.. index:: PHP-API, TypoScript, YAML, PartiallyScanned, ext:frontend
diff --git a/typo3/sysext/core/Tests/Acceptance/Application/Site/SiteModuleCest.php b/typo3/sysext/core/Tests/Acceptance/Application/Site/SiteModuleCest.php
index bbe2b3f4208406efc93363c54c9f9f9451184b91..150c346c84ccadd7bf1c6c6c29b03f24f8a35c72 100644
--- a/typo3/sysext/core/Tests/Acceptance/Application/Site/SiteModuleCest.php
+++ b/typo3/sysext/core/Tests/Acceptance/Application/Site/SiteModuleCest.php
@@ -84,8 +84,7 @@ final class SiteModuleCest
         $I->canSee('[New language]');
         $I->fillField('//input[contains(@data-formengine-input-name, "data[site_language]") and contains(@data-formengine-input-name, "[title]")]', 'New Language');
         $I->fillField('//input[contains(@data-formengine-input-name, "data[site_language]") and contains(@data-formengine-input-name, "[base]")]', '/new-language/');
-        $I->fillField('//input[contains(@data-formengine-input-name, "data[site_language]") and contains(@data-formengine-input-name, "[locale]")]', 'C');
-        $I->selectOption('//select[contains(@name, "[site_language]") and contains(@name, "[iso-639-1]")]', 'hr');
+        $I->fillField('//input[contains(@data-formengine-input-name, "data[site_language]") and contains(@data-formengine-input-name, "[locale]")]', 'hr_HR');
 
         $I->amGoingTo('Save the site configuration');
         $I->waitForElement($saveButtonLink, 30);
@@ -93,7 +92,7 @@ final class SiteModuleCest
         $I->waitForElementNotVisible('#t3js-ui-block');
 
         $I->amGoingTo('Verify new site configuration has been added with the next available language ID)');
-        $I->canSee('New Language [9] (C)');
+        $I->canSee('New Language [9] (hr_HR)');
 
         $I->amGoingTo('Close the site configuration form');
         $I->click('Close');
@@ -116,9 +115,9 @@ final class SiteModuleCest
         $I->selectOption('.t3js-create-new-selector', '9');
         $I->waitForElementVisible('div.inlineIsNewRecord:nth-child(2)');
         $I->scrollTo('div.inlineIsNewRecord:nth-child(2)');
-        $I->canSee('New Language [9] (C)');
+        $I->canSee('New Language [9] (hr_HR)');
         $I->click('div.inlineIsNewRecord:nth-child(2) > div:nth-child(1) > div:nth-child(1)');
-        $I->canSee('/hr/');
+        $I->canSee('/hr-hr/');
         $I->click('div.inlineIsNewRecord:nth-child(2) > div:nth-child(1) > div:nth-child(1) > div:nth-child(3) button');
         $modalDialog->canSeeDialog();
         $I->click('button[name="yes"]', ModalDialog::$openedModalButtonContainerSelector);
diff --git a/typo3/sysext/core/Tests/Functional/DataHandling/AbstractDataHandlerActionTestCase.php b/typo3/sysext/core/Tests/Functional/DataHandling/AbstractDataHandlerActionTestCase.php
index e0db566612ed99205a77e11f5e56c97daff2cc8f..264e7476844f0ef33113d3de9eec6aa605cab601 100644
--- a/typo3/sysext/core/Tests/Functional/DataHandling/AbstractDataHandlerActionTestCase.php
+++ b/typo3/sysext/core/Tests/Functional/DataHandling/AbstractDataHandlerActionTestCase.php
@@ -70,7 +70,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
             'languageId' => 1,
             'base' => '/dk/',
             'locale' => 'da_DK.UTF-8',
-            'iso-639-1' => 'da',
             'flag' => 'dk',
             'fallbackType' => 'fallback',
             'fallbacks' => '0',
@@ -81,7 +80,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
             'languageId' => 2,
             'base' => '/de/',
             'locale' => 'de_DE.UTF-8',
-            'iso-639-1' => 'de',
             'flag' => 'de',
             'fallbackType' => 'fallback',
             'fallbacks' => '1,0',
@@ -123,7 +121,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
                 'languageId' => 0,
                 'base' => '/',
                 'locale' => 'en_US.UTF-8',
-                'iso-639-1' => 'en',
                 'navigationTitle' => '',
                 'hreflang' => '',
                 'direction' => '',
diff --git a/typo3/sysext/core/Tests/Functional/DataHandling/Regular/Hooks/PagesTsConfigGuardTest.php b/typo3/sysext/core/Tests/Functional/DataHandling/Regular/Hooks/PagesTsConfigGuardTest.php
index e93d6901be70790368e101ec6fd62db7869e4311..a83c31a6e63e6190a29f4aeb654465da1488b994 100644
--- a/typo3/sysext/core/Tests/Functional/DataHandling/Regular/Hooks/PagesTsConfigGuardTest.php
+++ b/typo3/sysext/core/Tests/Functional/DataHandling/Regular/Hooks/PagesTsConfigGuardTest.php
@@ -118,7 +118,6 @@ class PagesTsConfigGuardTest extends FunctionalTestCase
                     'languageId' => 0,
                     'base' => '/',
                     'locale' => 'en_US.UTF-8',
-                    'iso-639-1' => 'en',
                     'navigationTitle' => '',
                     'hreflang' => '',
                     'direction' => '',
diff --git a/typo3/sysext/core/Tests/Functional/DataHandling/Slug/SlugHelperTest.php b/typo3/sysext/core/Tests/Functional/DataHandling/Slug/SlugHelperTest.php
index 894ccc57aebb9794b6ef7ced79ff24b1ab88329b..bdea480a2db63b89f071ec5826a4cc0766dfe8c9 100644
--- a/typo3/sysext/core/Tests/Functional/DataHandling/Slug/SlugHelperTest.php
+++ b/typo3/sysext/core/Tests/Functional/DataHandling/Slug/SlugHelperTest.php
@@ -37,7 +37,6 @@ class SlugHelperTest extends AbstractDataHandlerActionTestCase
             'languageId' => 1,
             'base' => '/dk/',
             'locale' => 'da_DK.UTF-8',
-            'iso-639-1' => 'da',
             'flag' => 'dk',
             'fallbackType' => 'fallback',
             'fallbacks' => '0',
@@ -48,7 +47,6 @@ class SlugHelperTest extends AbstractDataHandlerActionTestCase
             'languageId' => 2,
             'base' => '/de/',
             'locale' => 'de_DE.UTF-8',
-            'iso-639-1' => 'de',
             'flag' => 'de',
             'fallbackType' => 'fallback',
             'fallbacks' => '0',
@@ -59,7 +57,6 @@ class SlugHelperTest extends AbstractDataHandlerActionTestCase
             'languageId' => 3,
             'base' => '/de-ch/',
             'locale' => 'de_CH.UTF-8',
-            'iso-639-1' => 'de',
             'flag' => 'ch',
             'fallbackType' => 'fallback',
             'fallbacks' => '2,0',
diff --git a/typo3/sysext/core/Tests/Functional/DataScenarios/AbstractDataHandlerActionTestCase.php b/typo3/sysext/core/Tests/Functional/DataScenarios/AbstractDataHandlerActionTestCase.php
index de1cea89febe20c0ca9646d6b7ec4cc6248a0a9e..59d8367f910bcee34fdf9632cf57f9918d7ccde7 100644
--- a/typo3/sysext/core/Tests/Functional/DataScenarios/AbstractDataHandlerActionTestCase.php
+++ b/typo3/sysext/core/Tests/Functional/DataScenarios/AbstractDataHandlerActionTestCase.php
@@ -70,7 +70,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
             'languageId' => 1,
             'base' => '/dk/',
             'locale' => 'da_DK.UTF-8',
-            'iso-639-1' => 'da',
             'flag' => 'dk',
             'fallbackType' => 'fallback',
             'fallbacks' => '0',
@@ -81,7 +80,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
             'languageId' => 2,
             'base' => '/de/',
             'locale' => 'de_DE.UTF-8',
-            'iso-639-1' => 'de',
             'flag' => 'de',
             'fallbackType' => 'fallback',
             'fallbacks' => '1,0',
@@ -123,7 +121,6 @@ abstract class AbstractDataHandlerActionTestCase extends FunctionalTestCase
                 'languageId' => 0,
                 'base' => '/',
                 'locale' => 'en_US.UTF-8',
-                'iso-639-1' => 'en',
                 'navigationTitle' => '',
                 'hreflang' => '',
                 'direction' => '',
diff --git a/typo3/sysext/core/Tests/Functional/SiteHandling/SiteBasedTestTrait.php b/typo3/sysext/core/Tests/Functional/SiteHandling/SiteBasedTestTrait.php
index 30e84d08048714970d0947311f240c76d8a202db..47befee7e7e2ca3ba03dff7e193cef3d23c33cf1 100644
--- a/typo3/sysext/core/Tests/Functional/SiteHandling/SiteBasedTestTrait.php
+++ b/typo3/sysext/core/Tests/Functional/SiteHandling/SiteBasedTestTrait.php
@@ -127,7 +127,6 @@ trait SiteBasedTestTrait
             'navigationTitle' => $preset['title'],
             'base' => $base,
             'locale' => $preset['locale'],
-            'iso-639-1' => $preset['iso'] ?? '',
             'hreflang' => $preset['hrefLang'] ?? '',
             'direction' => $preset['direction'] ?? '',
             'flag' => $preset['iso'] ?? '',
diff --git a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1.yaml b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1.yaml
index 0e88fb572947e95ccb9d042cbce7045b87fad6e7..6d8278de16c0c93a94b2c8c22a26e93d145a4148 100644
--- a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1.yaml
+++ b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1.yaml
@@ -16,7 +16,6 @@ languages:
     languageId: '0'
     base: /en
     locale: en_US.utf8
-    iso-639-1: en
     hreflang: ''
     direction: ''
     flag: bv
@@ -27,7 +26,6 @@ languages:
     languageId: '1'
     base: /da
     locale: da_DK.utf8
-    iso-639-1: da
     hreflang: ''
     direction: ''
     flag: da
diff --git a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1_expected.yaml b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1_expected.yaml
index 5ad310305c8d8067fa31e6c28b27a4642c5a967d..037f7d48da51d558e5c0438734ba4ddb7b6a6ac0 100644
--- a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1_expected.yaml
+++ b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config1_expected.yaml
@@ -15,7 +15,6 @@ languages:
     languageId: '0'
     base: /en
     locale: en_US.utf8
-    iso-639-1: en
     hreflang: ''
     direction: ''
     flag: bv
diff --git a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2.yaml b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2.yaml
index aa665df0563b7aba72106633cab8aa8bac1207eb..265b26e535233c0750d6ed2c16fdb6ff645269dd 100644
--- a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2.yaml
+++ b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2.yaml
@@ -16,7 +16,6 @@ languages:
     languageId: '1'
     base: /da
     locale: da_DK.utf8
-    iso-639-1: da
     hreflang: ''
     direction: ''
     flag: da
diff --git a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2_expected.yaml b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2_expected.yaml
index bd9e38e30777c932043a78c9c9c8c0a51263d9f4..97e63c279121f2355e1882142940a696d6847af5 100644
--- a/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2_expected.yaml
+++ b/typo3/sysext/core/Tests/Unit/Configuration/Fixtures/SiteConfigs/config2_expected.yaml
@@ -16,7 +16,6 @@ languages:
     languageId: '0'
     base: /en
     locale: en_US.utf8
-    iso-639-1: en
     hreflang: ''
     direction: ''
     flag: en
@@ -27,7 +26,6 @@ languages:
     languageId: '1'
     base: /da
     locale: da_DK.utf8
-    iso-639-1: da
     hreflang: ''
     direction: ''
     flag: da
diff --git a/typo3/sysext/core/Tests/Unit/Configuration/SiteConfigurationTest.php b/typo3/sysext/core/Tests/Unit/Configuration/SiteConfigurationTest.php
index 2eaff4b3ef126f6df6ec549157dab3d583d0f61c..0aacc54927c2a47769e91c9ef566142ec2f3018a 100644
--- a/typo3/sysext/core/Tests/Unit/Configuration/SiteConfigurationTest.php
+++ b/typo3/sysext/core/Tests/Unit/Configuration/SiteConfigurationTest.php
@@ -139,7 +139,6 @@ class SiteConfigurationTest extends UnitTestCase
             'languageId' => '0',
             'base' => '/en',
             'locale' => 'en_US.utf8',
-            'iso-639-1' => 'en',
             'hreflang' => '',
             'direction' => '',
             'flag' => 'en',
diff --git a/typo3/sysext/core/Tests/Unit/Site/Entity/SiteLanguageTest.php b/typo3/sysext/core/Tests/Unit/Site/Entity/SiteLanguageTest.php
index ff0fb0989f4d3c5096a44401f7a3e7bb6a3e5e69..a680b795f4ecb218f756cf402215aa343dc2957e 100644
--- a/typo3/sysext/core/Tests/Unit/Site/Entity/SiteLanguageTest.php
+++ b/typo3/sysext/core/Tests/Unit/Site/Entity/SiteLanguageTest.php
@@ -97,7 +97,7 @@ class SiteLanguageTest extends UnitTestCase
             'base' => '/',
             'title' => 'Default',
             'websiteTitle' => '',
-            'twoLetterIsoCode' => 'en',
+            'twoLetterIsoCode' => 'de',
             'hreflang' => '',
             'direction' => '',
             'typo3Language' => 'de',
diff --git a/typo3/sysext/form/Classes/Controller/FormEditorController.php b/typo3/sysext/form/Classes/Controller/FormEditorController.php
index 98e5058b2d4b21c44f5d97e98230ddaa6636729a..07f7b8ca01d49dce01e1adafdbfc756b85591076 100644
--- a/typo3/sysext/form/Classes/Controller/FormEditorController.php
+++ b/typo3/sysext/form/Classes/Controller/FormEditorController.php
@@ -291,7 +291,6 @@ class FormEditorController extends AbstractBackendController
                     'navigationTitle' => '',
                     'flag' => '',
                     'locale' => '',
-                    'iso-639-1' => '',
                     'hreflang' => '',
                     'direction' => '',
                 ],
diff --git a/typo3/sysext/form/Classes/Domain/Runtime/FormRuntime.php b/typo3/sysext/form/Classes/Domain/Runtime/FormRuntime.php
index 5d9068ff4558dc37d8dd4b63d4e1b66902bb0012..1be3650e6c31a549f3535a928cc9ee9c7590821b 100644
--- a/typo3/sysext/form/Classes/Domain/Runtime/FormRuntime.php
+++ b/typo3/sysext/form/Classes/Domain/Runtime/FormRuntime.php
@@ -1049,7 +1049,6 @@ class FormRuntime implements RootRenderableInterface, \ArrayAccess
                         'navigationTitle' => '',
                         'flag' => '',
                         'locale' => '',
-                        'iso-639-1' => '',
                         'hreflang' => '',
                         'direction' => '',
                     ],
diff --git a/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php b/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php
index d62d14cc8433b5c3e4d6341ec31272f9599879db..6e937613785e2886361c7a3477f4e105f0e82323 100644
--- a/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php
+++ b/typo3/sysext/frontend/Classes/ContentObject/ContentObjectRenderer.php
@@ -4139,9 +4139,33 @@ class ContentObjectRenderer implements LoggerAwareInterface
                         break;
                     case 'sitelanguage':
                         $siteLanguage = $this->getTypoScriptFrontendController()->getLanguage();
-                        $config = $siteLanguage->toArray();
-                        if (isset($config[$key])) {
-                            $retVal = $config[$key] ?? '';
+                        if ($key === 'twoLetterIsoCode') {
+                            $key = 'locale:languageCode';
+                        }
+                        // Special handling for the locale object
+                        if (str_starts_with($key, 'locale')) {
+                            $localeObject = $siteLanguage->getLocale();
+                            if ($key === 'locale') {
+                                // backwards-compatibility
+                                $retVal = $localeObject->posixFormatted();
+                            } else {
+                                $keyParts = explode(':', $key, 2);
+                                switch ($keyParts[1] ?? '') {
+                                    case 'languageCode':
+                                        $retVal = $localeObject->getLanguageCode();
+                                        break;
+                                    case 'countryCode':
+                                        $retVal = $localeObject->getCountryCode();
+                                        break;
+                                    default:
+                                        $retVal = $localeObject->getName();
+                                }
+                            }
+                        } else {
+                            $config = $siteLanguage->toArray();
+                            if (isset($config[$key])) {
+                                $retVal = $config[$key] ?? '';
+                            }
                         }
                         break;
                     case 'sitesettings':
diff --git a/typo3/sysext/frontend/Tests/Functional/Rendering/TitleTagRenderingTest.php b/typo3/sysext/frontend/Tests/Functional/Rendering/TitleTagRenderingTest.php
index 47c4303c0738e837a41fea8ddd7c1d7732c1ace8..e8633cc52132d35c7b5191c2dd4b24064652a6c1 100644
--- a/typo3/sysext/frontend/Tests/Functional/Rendering/TitleTagRenderingTest.php
+++ b/typo3/sysext/frontend/Tests/Functional/Rendering/TitleTagRenderingTest.php
@@ -62,7 +62,6 @@ class TitleTagRenderingTest extends FunctionalTestCase
                     'languageId' => '0',
                     'base' => '/',
                     'locale' => 'en_US.UTF-8',
-                    'iso-639-1' => 'en',
                     'websiteTitle' => 'Site EN',
                     'navigationTitle' => '',
                     'hreflang' => '',
diff --git a/typo3/sysext/frontend/Tests/Functional/SiteHandling/EnhancerLinkGenerator/RouteTest.php b/typo3/sysext/frontend/Tests/Functional/SiteHandling/EnhancerLinkGenerator/RouteTest.php
index 150a700c018cecfad0410479db8259376d4ba1ec..fb46bb04944eed68bc52618257b450ef07355602 100644
--- a/typo3/sysext/frontend/Tests/Functional/SiteHandling/EnhancerLinkGenerator/RouteTest.php
+++ b/typo3/sysext/frontend/Tests/Functional/SiteHandling/EnhancerLinkGenerator/RouteTest.php
@@ -85,7 +85,7 @@ class RouteTest extends AbstractEnhancerLinkGeneratorTestCase
                         ],
                         'localeMap' => [
                             [
-                                'locale' => 'fr_FR',
+                                'locale' => 'fr-FR',
                                 'map' => [
                                     'cent' => 100,
                                 ],
@@ -180,7 +180,7 @@ class RouteTest extends AbstractEnhancerLinkGeneratorTestCase
                         ],
                         'localeMap' => [
                             [
-                                'locale' => 'fr_FR',
+                                'locale' => 'fr-FR',
                                 'map' => [
                                     'cent' => 100,
                                 ],
diff --git a/typo3/sysext/frontend/Tests/Unit/ContentObject/ContentObjectRendererTest.php b/typo3/sysext/frontend/Tests/Unit/ContentObject/ContentObjectRendererTest.php
index 033dfb1117065cfc4a43058df73234745822fcdb..a1d463ba88b6fdc84847acc83cdfe3cf4e8efd41 100644
--- a/typo3/sysext/frontend/Tests/Unit/ContentObject/ContentObjectRendererTest.php
+++ b/typo3/sysext/frontend/Tests/Unit/ContentObject/ContentObjectRendererTest.php
@@ -1576,6 +1576,8 @@ class ContentObjectRendererTest extends UnitTestCase
         $language = $site->getLanguageById(1);
         $this->frontendControllerMock->_set('language', $language);
         self::assertEquals('German', $this->subject->getData('siteLanguage:navigationTitle'));
+        self::assertEquals('de', $this->subject->getData('siteLanguage:twoLetterIsoCode'));
+        self::assertEquals('de', $this->subject->getData('siteLanguage:locale:languageCode'));
     }
 
     /**
diff --git a/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodCallMatcher.php b/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodCallMatcher.php
index c658d05abf08bde21a12a3a4d95228f9dc032fc2..6c63e6fe8119f3d13cc0465f505028ac6839de6f 100644
--- a/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodCallMatcher.php
+++ b/typo3/sysext/install/Configuration/ExtensionScanner/Php/MethodCallMatcher.php
@@ -5491,4 +5491,11 @@ return [
             'Deprecation-99685-RemoveLineBreaksFromTemplate.rst',
         ],
     ],
+    'TYPO3\CMS\Core\Site\Entity\SiteLanguage->getTwoLetterIsoCode' => [
+        'numberOfMandatoryArguments' => 0,
+        'maximumNumberOfArguments' => 0,
+        'restFiles' => [
+            'Deprecation-99905-SiteLanguageIso-639-1Setting.rst',
+        ],
+    ],
 ];
diff --git a/typo3/sysext/install/Tests/Functional/Updates/MigrateSiteSettingsConfigUpdateTest.php b/typo3/sysext/install/Tests/Functional/Updates/MigrateSiteSettingsConfigUpdateTest.php
index 69f8b3a2554ce0a15eb1ff4028058b14676402a0..a7bc381b7616fd660a8973f625d23fd90292510f 100644
--- a/typo3/sysext/install/Tests/Functional/Updates/MigrateSiteSettingsConfigUpdateTest.php
+++ b/typo3/sysext/install/Tests/Functional/Updates/MigrateSiteSettingsConfigUpdateTest.php
@@ -44,7 +44,6 @@ class MigrateSiteSettingsConfigUpdateTest extends FunctionalTestCase
                         'languageId' => 0,
                         'base' => '/',
                         'locale' => 'en_US.UTF-8',
-                        'iso-639-1' => 'en',
                         'navigationTitle' => 'English',
                         'hreflang' => 'en-us',
                         'direction' => 'ltr',
@@ -84,7 +83,6 @@ class MigrateSiteSettingsConfigUpdateTest extends FunctionalTestCase
                         'languageId' => 0,
                         'base' => '/',
                         'locale' => 'en_US.UTF-8',
-                        'iso-639-1' => 'en',
                         'navigationTitle' => 'English',
                         'hreflang' => 'en-us',
                         'direction' => 'ltr',
diff --git a/typo3/sysext/linkvalidator/Classes/Result/LinkAnalyzerResult.php b/typo3/sysext/linkvalidator/Classes/Result/LinkAnalyzerResult.php
index e89a39d55380b525caac5062b600621c4182dff0..f2237eea17292cf1ac62a5f1738619dc17f18d1e 100644
--- a/typo3/sysext/linkvalidator/Classes/Result/LinkAnalyzerResult.php
+++ b/typo3/sysext/linkvalidator/Classes/Result/LinkAnalyzerResult.php
@@ -204,7 +204,7 @@ class LinkAnalyzerResult
 
             try {
                 $site = GeneralUtility::makeInstance(SiteFinder::class)->getSiteByPageId((int)$brokenLink['real_pid']);
-                $languageCode = $site->getLanguageById((int)$brokenLink['language'])->getTwoLetterIsoCode() ?: 'default';
+                $languageCode = $site->getLanguageById((int)$brokenLink['language'])->getLocale()->getLanguageCode();
             } catch (SiteNotFoundException | \InvalidArgumentException $e) {
                 $languageCode = 'default';
             }
@@ -214,7 +214,7 @@ class LinkAnalyzerResult
 
             $brokenLink['record_type'] = $this->getLanguageService()->sL($GLOBALS['TCA'][$brokenLink['table_name']]['ctrl']['title'] ?? '');
             $brokenLink['target'] = (((string)($brokenLink['link_type'] ?? '') === 'db') ? 'id:' : '') . $brokenLink['url'];
-            $brokenLink['language_code'] = $languageCode;
+            $brokenLink['language_code'] = (string)$languageCode;
         }
 
         return $this;
diff --git a/typo3/sysext/redirects/Tests/Functional/Service/SlugServiceTest.php b/typo3/sysext/redirects/Tests/Functional/Service/SlugServiceTest.php
index 336df32a3dc4c752999ce65ed38ec658f64c73fe..33ea4eda6e56803f991afb24ba270c25a3bc15bd 100644
--- a/typo3/sysext/redirects/Tests/Functional/Service/SlugServiceTest.php
+++ b/typo3/sysext/redirects/Tests/Functional/Service/SlugServiceTest.php
@@ -58,7 +58,6 @@ class SlugServiceTest extends FunctionalTestCase
             'languageId' => '0',
             'base' => '/en/',
             'locale' => 'en_US.UTF-8',
-            'iso-639-1' => 'en',
             'navigationTitle' => 'English',
             'hreflang' => 'en-us',
             'direction' => 'ltr',
@@ -70,7 +69,6 @@ class SlugServiceTest extends FunctionalTestCase
             'languageId' => '1',
             'base' => 'https://de.example.com/',
             'locale' => 'de_DE.UTF-8',
-            'iso-639-1' => 'de',
             'navigationTitle' => 'German',
             'hreflang' => 'de-de',
             'direction' => 'ltr',
@@ -82,7 +80,6 @@ class SlugServiceTest extends FunctionalTestCase
             'languageId' => '2',
             'base' => '/es/',
             'locale' => 'es_ES.UTF-8',
-            'iso-639-1' => 'es',
             'navigationTitle' => 'Spanish',
             'hreflang' => 'es-es',
             'direction' => 'ltr',