diff --git a/src/V2/Localization/Exception/InvalidLanguageException.php b/src/V2/Localization/Exception/InvalidLanguageException.php
new file mode 100644
index 0000000..53d621f
--- /dev/null
+++ b/src/V2/Localization/Exception/InvalidLanguageException.php
@@ -0,0 +1,47 @@
+<?php
+
+namespace GW2Treasures\GW2Api\V2\Localization\Exception;
+
+use GuzzleHttp\Message\ResponseInterface;
+use GW2Treasures\GW2Api\Exception\ApiException;
+
+class InvalidLanguageException extends ApiException {
+
+    /** @var string $requestLanguage */
+    protected $requestLanguage;
+
+    /** @var string $responseLanguage */
+    protected $responseLanguage;
+
+    /**
+     * @param string            $message
+     * @param string            $requestLanguage
+     * @param string            $responseLanguage
+     * @param ResponseInterface $response
+     */
+    public function __construct( $message, $requestLanguage, $responseLanguage, ResponseInterface $response ) {
+        $this->requestLanguage = $requestLanguage;
+        $this->responseLanguage = $responseLanguage;
+
+        parent::__construct( $message, $response );
+    }
+
+    /**
+     * Get the requested language.
+     *
+     * @return string
+     */
+    public function getRequestLanguage() {
+        return $this->requestLanguage;
+    }
+
+    /**
+     * Get the language the api responded with.
+     *
+     * @return string
+     */
+    public function getResponseLanguage() {
+        return $this->responseLanguage;
+    }
+
+}
diff --git a/src/V2/Localization/LocalizationHandler.php b/src/V2/Localization/LocalizationHandler.php
index dcaf4ed..5edf0df 100644
--- a/src/V2/Localization/LocalizationHandler.php
+++ b/src/V2/Localization/LocalizationHandler.php
@@ -3,7 +3,9 @@
 namespace GW2Treasures\GW2Api\V2\Localization;
 
 use GuzzleHttp\Message\RequestInterface;
+use GuzzleHttp\Message\ResponseInterface;
 use GW2Treasures\GW2Api\V2\ApiHandler;
+use GW2Treasures\GW2Api\V2\Localization\Exception\InvalidLanguageException;
 
 class LocalizationHandler extends ApiHandler {
     function __construct( ILocalizedEndpoint $endpoint ) {
@@ -27,4 +29,14 @@ protected function getEndpoint() {
     public function onRequest( RequestInterface $request ) {
         $request->getQuery()->add( 'lang', $this->getEndpoint()->getLang() );
     }
+
+    public function onResponse( ResponseInterface $response, RequestInterface $request ) {
+        $requestLanguage = $request->getQuery()->get('lang');
+        $responseLanguage = $response->getHeader( 'Content-Language' );
+
+        if( $requestLanguage !== $responseLanguage ) {
+            $message = 'Invalid language (expected: ' . $requestLanguage . '; actual: ' . $responseLanguage . ')';
+            throw new InvalidLanguageException( $message, $requestLanguage, $responseLanguage, $response );
+        }
+    }
 }
diff --git a/tests/LocalizedEndpointTest.php b/tests/LocalizedEndpointTest.php
index 90262cc..73bb826 100644
--- a/tests/LocalizedEndpointTest.php
+++ b/tests/LocalizedEndpointTest.php
@@ -1,5 +1,6 @@
 <?php
 
+use GW2Treasures\GW2Api\V2\Localization\Exception\InvalidLanguageException;
 use Stubs\LocalizedEndpointStub;
 
 class LocalizedEndpointTest extends TestCase {
@@ -20,8 +21,8 @@ public function testBasic() {
     }
 
     public function testRepeated() {
-        $this->mockResponse('[]');
-        $this->mockResponse('[]');
+        $this->mockResponse('[]', 'de');
+        $this->mockResponse('[]', 'de');
 
         $endpoint_de = $this->getLocalizedEndpoint()->lang('de');
 
@@ -36,7 +37,7 @@ public function testRepeated() {
     }
 
     public function testNested() {
-        $this->mockResponse('[]');
+        $this->mockResponse('[]', 'fr');
 
         $this->getLocalizedEndpoint()->lang('es')->lang('fr')->test();
 
@@ -46,4 +47,21 @@ public function testNested() {
         $this->assertEquals( 'fr', $request->getQuery()->get('lang'),
             'LocalizedEndpoint sets correct query parameter value on nested request' );
     }
+
+    public function testInvalidLanguage() {
+        $this->mockResponse('[]', 'en');
+
+        try {
+            $this->getLocalizedEndpoint()->lang('invalid')->test();
+        } catch( InvalidLanguageException $ex ) {
+            $this->assertEquals( 'en', $ex->getResponseLanguage() );
+            $this->assertEquals( 'invalid', $ex->getRequestLanguage() );
+
+            $this->assertTrue( strstr( $ex->getMessage(), 'Invalid language' ) !== false );
+
+            return;
+        }
+
+        $this->fail();
+    }
 }
diff --git a/tests/TestCase.php b/tests/TestCase.php
index 5b629c4..fdf2838 100644
--- a/tests/TestCase.php
+++ b/tests/TestCase.php
@@ -29,7 +29,7 @@ protected function api() {
     /**
      * @param Response|RequestException|string $response
      */
-    protected function mockResponse( $response ) {
+    protected function mockResponse( $response, $language = 'en' ) {
         if( !isset( $this->mock )) {
             $this->mock = new Mock();
             $this->history = new History();
@@ -38,7 +38,7 @@ protected function mockResponse( $response ) {
         }
         if( is_string( $response )) {
             $this->mock->addResponse(
-                new Response( 200, ['Content-Type' => 'application/json; charset=utf-8'], Stream::factory( $response ))
+                new Response( 200, ['Content-Type' => 'application/json; charset=utf-8', 'Content-Language' => $language], Stream::factory( $response ))
             );
         } elseif( $response instanceof RequestException ) {
             $this->mock->addException( $response );