reverted:
--- b/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
+++ a/core/modules/ckeditor/src/Plugin/CKEditorPlugin/Internal.php
@@ -11,7 +11,6 @@
 use Drupal\Component\Utility\NestedArray;
 use Drupal\Core\Cache\Cache;
 use Drupal\Core\Cache\CacheBackendInterface;
-use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\editor\Entity\Editor;
 use Drupal\filter\Plugin\FilterInterface;
@@ -326,7 +325,7 @@
       $possible_format_tags = array('h1', 'h2', 'h3', 'h4', 'h5', 'h6', 'pre');
       foreach ($possible_format_tags as $tag) {
         $input = '<' . $tag . '>TEST</' . $tag . '>';
+        $output = trim(check_markup($input, $editor->id()));
-        $output = trim(check_markup($input, $editor->id(), LanguageInterface::LANGCODE_NOT_SPECIFIED));
         if ($input == $output) {
           $format_tags[] = $tag;
         }
diff -u b/core/modules/filter/filter.module b/core/modules/filter/filter.module
--- b/core/modules/filter/filter.module
+++ b/core/modules/filter/filter.module
@@ -11,6 +11,7 @@
 use Drupal\Component\Utility\String;
 use Drupal\Component\Utility\Xss;
 use Drupal\Core\Cache\Cache;
+use Drupal\Core\Language\LanguageInterface;
 use Drupal\Core\Render\Element;
 use Drupal\Core\Routing\RouteMatchInterface;
 use Drupal\Core\Session\AccountInterface;
@@ -445,9 +446,9 @@
  * @param string $format_id
  *   The machine name of the filter format to be used to filter the text.
  * @param string $langcode
- *   The language code of the text to be filtered, e.g. 'en' for English. This
- *   allows filters to be language-aware so language-specific text replacement
- *   can be implemented.
+ *   (optional) The language code of the text to be filtered, e.g. 'en' for
+ *   English. This allows filters to be language-aware so language-specific text
+ *   replacement can be implemented. Defaults to 'und'.
  * @param array $filter_types_to_skip
  *   (optional) An array of filter types to skip, or an empty array (default)
  *   to skip no filter types. All of the format's filters will be applied,
@@ -462,7 +463,7 @@
  *
  * @ingroup sanitization
  */
-function check_markup($text, $format_id, $langcode, $filter_types_to_skip = array()) {
+function check_markup($text, $format_id, $langcode = LanguageInterface::LANGCODE_NOT_SPECIFIED, $filter_types_to_skip = array()) {
   $build = array(
     '#type' => 'processed_text',
     '#text' => $text,
diff -u b/core/modules/filter/src/Tests/FilterAPITest.php b/core/modules/filter/src/Tests/FilterAPITest.php
--- b/core/modules/filter/src/Tests/FilterAPITest.php
+++ b/core/modules/filter/src/Tests/FilterAPITest.php
@@ -59,7 +59,7 @@
     $text = "<p>Llamas are <not> awesome!</p>";
     $expected_filtered_text = "&lt;p&gt;Llamas are  awesome!&lt;/p&gt;";
 
-    $this->assertIdentical(check_markup($text, 'crazy', LanguageInterface::LANGCODE_NOT_SPECIFIED), $expected_filtered_text, 'Filters applied in correct order.');
+    $this->assertIdentical(check_markup($text, 'crazy'), $expected_filtered_text, 'Filters applied in correct order.');
   }
 
   /**
diff -u b/core/modules/filter/src/Tests/FilterNoFormatTest.php b/core/modules/filter/src/Tests/FilterNoFormatTest.php
--- b/core/modules/filter/src/Tests/FilterNoFormatTest.php
+++ b/core/modules/filter/src/Tests/FilterNoFormatTest.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\filter\Tests;
 
-use Drupal\Core\Language\LanguageInterface;
 use Drupal\simpletest\WebTestBase;
 
 /**
@@ -38,5 +37,5 @@
     // Make sure that when this text is run through check_markup() with no text
     // format, it is filtered as though it is in the fallback format.
-    $this->assertEqual(check_markup($text, NULL, LanguageInterface::LANGCODE_NOT_SPECIFIED), check_markup($text, filter_fallback_format(), LanguageInterface::LANGCODE_NOT_SPECIFIED), 'Text with no format is filtered the same as text in the fallback format.');
+    $this->assertEqual(check_markup($text, NULL), check_markup($text, filter_fallback_format()), 'Text with no format is filtered the same as text in the fallback format.');
   }
 }
reverted:
--- b/core/modules/views/src/Plugin/views/field/Markup.php
+++ a/core/modules/views/src/Plugin/views/field/Markup.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\views\Plugin\views\field;
 
-use Drupal\Core\Language\LanguageInterface;
 use Drupal\views\Plugin\views\display\DisplayPluginBase;
 use Drupal\views\ResultRow;
 use Drupal\views\ViewExecutable;
@@ -54,7 +53,7 @@
     }
     if ($value) {
       $value = str_replace('<!--break-->', '', $value);
+      return check_markup($value, $format);
-      return check_markup($value, $format, LanguageInterface::LANGCODE_NOT_SPECIFIED);
     }
   }
 
diff -u b/core/modules/views/src/Tests/Handler/AreaTextTest.php b/core/modules/views/src/Tests/Handler/AreaTextTest.php
--- b/core/modules/views/src/Tests/Handler/AreaTextTest.php
+++ b/core/modules/views/src/Tests/Handler/AreaTextTest.php
@@ -7,7 +7,6 @@
 
 namespace Drupal\views\Tests\Handler;
 
-use Drupal\Core\Language\LanguageInterface;
 use Drupal\views\Tests\ViewUnitTestBase;
 use Drupal\views\Views;
 
@@ -59,14 +58,14 @@
 
     $view->display_handler->handlers['header']['area']->options['format'] = filter_default_format();
     $build = $view->display_handler->handlers['header']['area']->render();
-    $this->assertEqual(check_markup($string, NULL, LanguageInterface::LANGCODE_NOT_SPECIFIED), drupal_render($build), 'Existent format should return something');
+    $this->assertEqual(check_markup($string, NULL), drupal_render($build), 'Existent format should return something');
 
     // Empty results, and it shouldn't be displayed .
     $this->assertEqual(array(), $view->display_handler->handlers['header']['area']->render(TRUE), 'No result should lead to no header');
     // Empty results, and it should be displayed.
     $view->display_handler->handlers['header']['area']->options['empty'] = TRUE;
     $build = $view->display_handler->handlers['header']['area']->render(TRUE);
-    $this->assertEqual(check_markup($string, NULL, LanguageInterface::LANGCODE_NOT_SPECIFIED), drupal_render($build), 'No result, but empty enabled lead to a full header');
+    $this->assertEqual(check_markup($string, NULL), drupal_render($build), 'No result, but empty enabled lead to a full header');
   }
 
 }