diff --git a/Classes/Service/LanguageService.php b/Classes/Service/LanguageService.php
index efd4570..e486ba1 100644
--- a/Classes/Service/LanguageService.php
+++ b/Classes/Service/LanguageService.php
@@ -13,8 +13,7 @@ public function getTargetLanguageForRephrasing(SiteInterface $site, int $targetL
     {
         $targetLanguage = $site->getLanguageById($targetLanguage);
         $targetLanguageConfiguration = $targetLanguage->toArray();
-        $deeplWriteLanguage = $targetLanguageConfiguration['deeplWriteLanguage'];
-
+        $deeplWriteLanguage = (string)($targetLanguageConfiguration['deeplWriteLanguage'] ?? '');
         return RephraseSupportedDeepLLanguage::tryFrom($deeplWriteLanguage);
     }
 }
diff --git a/Tests/Functional/Service/DeeplServiceTest.php b/Tests/Functional/Service/DeeplServiceTest.php
index c8908ad..bf04cc4 100644
--- a/Tests/Functional/Service/DeeplServiceTest.php
+++ b/Tests/Functional/Service/DeeplServiceTest.php
@@ -10,6 +10,10 @@
 
 final class DeeplServiceTest extends AbstractDeepLTestCase
 {
+    protected array $coreExtensionsToLoad = [
+        'typo3/cms-setup',
+    ];
+
     protected array $testExtensionsToLoad = [
         'web-vision/deepl-base',
         'web-vision/deepl-write',
diff --git a/Tests/Functional/Service/LanguageServiceTest.php b/Tests/Functional/Service/LanguageServiceTest.php
new file mode 100644
index 0000000..5953a7a
--- /dev/null
+++ b/Tests/Functional/Service/LanguageServiceTest.php
@@ -0,0 +1,164 @@
+<?php
+
+declare(strict_types=1);
+
+namespace WebVision\DeeplWrite\Tests\Functional\Service;
+
+use PHPUnit\Framework\Attributes\Test;
+use TYPO3\CMS\Core\Site\Entity\Site;
+use WebVision\DeeplWrite\Service\LanguageService;
+use WebVision\DeeplWrite\Tests\Functional\Helper\AbstractDeepLTestCase;
+
+final class LanguageServiceTest extends AbstractDeepLTestCase
+{
+    protected array $coreExtensionsToLoad = [
+        'typo3/cms-setup',
+    ];
+
+    protected array $testExtensionsToLoad = [
+        'web-vision/deepl-base',
+        'web-vision/deepl-write',
+    ];
+
+    protected array $configurationToUseInTestInstance = [
+        'EXTENSIONS' => [
+            'deepl_write' => [
+                'apiKey' => 'mock_server',
+            ],
+        ],
+    ];
+
+    #[Test]
+    public function getTargetLanguageForRephrasingReturnsNullIfDeeplWriteLanguageDoesNotExistsInSiteConfig(): void
+    {
+        $site = new Site(
+            identifier: 'dummy',
+            rootPageId: 1,
+            configuration: [
+                'languages' => [
+                    [
+                        'languageId' => 0,
+                        'enabled' => true,
+                        'title' => 'English',
+                        'base' => '/',
+                        'locale' => 'en_US.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'default',
+                        'iso-639-1' => 'en',
+                        'hreflang' => 'en-us',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                    ],
+                    [
+                        'languageId' => 1,
+                        'enabled' => true,
+                        'title' => 'Deutsch',
+                        'base' => '/de/',
+                        'locale' => 'de_DE.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'de',
+                        'iso-639-1' => 'de',
+                        'hreflang' => 'de-de',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                    ],
+                ],
+            ],
+            settings: null,
+        );
+        $targetSite = $this->get(LanguageService::class)->getTargetLanguageForRephrasing($site, 1);
+        static::assertNull($targetSite);
+    }
+
+    #[Test]
+    public function getTargetLanguageForRephasingReturnsNullForInvalidConfiguredDeeplWriteLanguage(): void
+    {
+        $site = new Site(
+            identifier: 'dummy',
+            rootPageId: 1,
+            configuration: [
+                'languages' => [
+                    [
+                        'languageId' => 0,
+                        'enabled' => true,
+                        'title' => 'English',
+                        'base' => '/',
+                        'locale' => 'en_US.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'default',
+                        'iso-639-1' => 'en',
+                        'hreflang' => 'en-us',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                    ],
+                    [
+                        'languageId' => 1,
+                        'enabled' => true,
+                        'title' => 'Deutsch',
+                        'base' => '/de/',
+                        'locale' => 'de_DE.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'de',
+                        'iso-639-1' => 'de',
+                        'hreflang' => 'de-de',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                        'deeplWriteLanguage' => 'RU',
+                    ],
+                ],
+            ],
+            settings: null,
+        );
+        $targetSite = $this->get(LanguageService::class)->getTargetLanguageForRephrasing($site, 1);
+        static::assertNull($targetSite);
+    }
+
+    #[Test]
+    public function getTargetLanguageForRephrasingReturnsValidConfiguredDeeplWriteLanguage(): void
+    {
+        $site = new Site(
+            identifier: 'dummy',
+            rootPageId: 1,
+            configuration: [
+                'languages' => [
+                    [
+                        'languageId' => 0,
+                        'enabled' => true,
+                        'title' => 'English',
+                        'base' => '/',
+                        'locale' => 'en_US.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'default',
+                        'iso-639-1' => 'en',
+                        'hreflang' => 'en-us',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                    ],
+                    [
+                        'languageId' => 1,
+                        'enabled' => true,
+                        'title' => 'Deutsch',
+                        'base' => '/de/',
+                        'locale' => 'de_DE.UTF-8',
+                        'navigationTitle' => 'English',
+                        'flag' => 'us',
+                        'typo3Language' => 'de',
+                        'iso-639-1' => 'de',
+                        'hreflang' => 'de-de',
+                        'direction' => 'ltr',
+                        'websiteTitle' => '',
+                        'deeplWriteLanguage' => 'DE',
+                    ],
+                ],
+            ],
+            settings: null,
+        );
+        $targetSite = $this->get(LanguageService::class)->getTargetLanguageForRephrasing($site, 1);
+        static::assertSame('DE', $targetSite);
+    }
+}