diff --git a/core/lib/Drupal/Core/Field/FormatterBase.php b/core/lib/Drupal/Core/Field/FormatterBase.php
index e428e41..880c80d 100644
--- a/core/lib/Drupal/Core/Field/FormatterBase.php
+++ b/core/lib/Drupal/Core/Field/FormatterBase.php
@@ -77,10 +77,8 @@ public function __construct($plugin_id, $plugin_definition, FieldDefinitionInter
   /**
    * {@inheritdoc}
    */
-  public function view(FieldItemListInterface $items, $langcode = NULL) {
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+  public function view(FieldItemListInterface $items, $langcode) {
+    $this->setMissingLangcode($langcode);
     $elements = $this->viewElements($items, $langcode);
 
     // If there are actual renderable children, use #theme => field, otherwise,
@@ -114,6 +112,15 @@ public function view(FieldItemListInterface $items, $langcode = NULL) {
   /**
    * {@inheritdoc}
    */
+  public function setMissingLangcode(&$langcode) {
+    if (empty($langcode)) {
+      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
   public function settingsForm(array $form, FormStateInterface $form_state) {
     return array();
   }
diff --git a/core/lib/Drupal/Core/Field/FormatterInterface.php b/core/lib/Drupal/Core/Field/FormatterInterface.php
index d487c83..df931c4 100644
--- a/core/lib/Drupal/Core/Field/FormatterInterface.php
+++ b/core/lib/Drupal/Core/Field/FormatterInterface.php
@@ -70,13 +70,23 @@ public function prepareView(array $entities_items);
    * @param \Drupal\Core\Field\FieldItemListInterface $items
    *   The field values to be rendered.
    * @param string $langcode
-   *   (optional) The language that should be used to render the field. Defaults
-   *   to the current content language.
+   *   The language that should be used to render the field. Defaults to the
+   *   current content language.
    *
    * @return array
    *   A renderable array for a themed field with its label and all its values.
    */
-  public function view(FieldItemListInterface $items, $langcode = NULL);
+  public function view(FieldItemListInterface $items, $langcode);
+
+  /**
+   * Sets the language code variable to the current user interface language.
+   *
+   * @param string $langcode
+   *   The language code passed by reference. If it is null/empty, it is set to
+   *   the current user interface language. If it is already defined, nothing is
+   *   done.
+   */
+  public function setMissingLangcode(&$langcode);
 
   /**
    * Builds a renderable array for a field value.
@@ -91,7 +101,7 @@ public function view(FieldItemListInterface $items, $langcode = NULL);
    *   A renderable array for $items, as an array of child elements keyed by
    *   consecutive numeric indexes starting from 0.
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL);
+  public function viewElements(FieldItemListInterface $items, $langcode);
 
   /**
    * Returns if the formatter can be used for the provided field.
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BasicStringFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BasicStringFormatter.php
index 9d15462..d72354c 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BasicStringFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BasicStringFormatter.php
@@ -31,7 +31,7 @@ class BasicStringFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BooleanFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BooleanFormatter.php
index f3fe774..2822905 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BooleanFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/BooleanFormatter.php
@@ -116,7 +116,7 @@ public function settingsForm(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     $formats = $this->getOutputFormats();
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceEntityFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceEntityFormatter.php
index 71d1236..53a7463 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceEntityFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceEntityFormatter.php
@@ -120,12 +120,10 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $view_mode = $this->getSetting('view_mode');
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
       // Protect ourselves from recursive rendering.
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
index 36cf3f6..5044fff 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
@@ -78,7 +78,7 @@ protected function getEntitiesToView(EntityReferenceFieldItemListInterface $item
    * @see ::prepareView()
    * @see ::getEntitiestoView()
    */
-  public function view(FieldItemListInterface $items, $langcode = NULL) {
+  public function view(FieldItemListInterface $items, $langcode) {
     $elements = parent::view($items, $langcode);
 
     $field_level_access_cacheability = new CacheableMetadata();
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceIdFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceIdFormatter.php
index 1d438d7..02dac34 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceIdFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceIdFormatter.php
@@ -27,11 +27,9 @@ class EntityReferenceIdFormatter extends EntityReferenceFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
       if ($entity->id()) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceLabelFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceLabelFormatter.php
index 3d4a16a..53c06c5 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceLabelFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/EntityReferenceLabelFormatter.php
@@ -60,12 +60,10 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
     $output_as_link = $this->getSetting('link');
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
       $label = $entity->label();
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/MailToFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/MailToFormatter.php
index bc9776a..94ab462 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/MailToFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/MailToFormatter.php
@@ -27,7 +27,7 @@ class MailToFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
index 7f3e1f8..0320cd5 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericFormatterBase.php
@@ -66,7 +66,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
     $settings = $this->getFieldSettings();
 
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericUnformattedFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericUnformattedFormatter.php
index 2a90e11..47e591b 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericUnformattedFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/NumericUnformattedFormatter.php
@@ -28,7 +28,7 @@ class NumericUnformattedFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/StringFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/StringFormatter.php
index f16163d..6fd862a 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/StringFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/StringFormatter.php
@@ -119,7 +119,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
     $url = NULL;
     if ($this->getSetting('link_to_entity')) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampAgoFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampAgoFormatter.php
index a5db296..26b01db 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampAgoFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampAgoFormatter.php
@@ -153,7 +153,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampFormatter.php
index 781de15..fbd44ac 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/TimestampFormatter.php
@@ -165,7 +165,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     $date_format = $this->getSetting('date_format');
diff --git a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/UriLinkFormatter.php b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/UriLinkFormatter.php
index 1f4ee96..bcf0a50 100644
--- a/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/UriLinkFormatter.php
+++ b/core/lib/Drupal/Core/Field/Plugin/Field/FieldFormatter/UriLinkFormatter.php
@@ -27,7 +27,7 @@ class UriLinkFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorTitleFormatter.php b/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorTitleFormatter.php
index b3c80bf..4de369d 100644
--- a/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorTitleFormatter.php
+++ b/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorTitleFormatter.php
@@ -54,7 +54,7 @@ public function settingsForm(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     if ($items->getEntity()->getEntityTypeId() == 'aggregator_feed') {
diff --git a/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorXSSFormatter.php b/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorXSSFormatter.php
index 42ca391..0edb460 100644
--- a/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorXSSFormatter.php
+++ b/core/modules/aggregator/src/Plugin/Field/FieldFormatter/AggregatorXSSFormatter.php
@@ -30,7 +30,7 @@ class AggregatorXSSFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/comment/src/Plugin/Field/FieldFormatter/AuthorNameFormatter.php b/core/modules/comment/src/Plugin/Field/FieldFormatter/AuthorNameFormatter.php
index 3e317d0..1d10dd0 100644
--- a/core/modules/comment/src/Plugin/Field/FieldFormatter/AuthorNameFormatter.php
+++ b/core/modules/comment/src/Plugin/Field/FieldFormatter/AuthorNameFormatter.php
@@ -28,7 +28,7 @@ class AuthorNameFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/comment/src/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php b/core/modules/comment/src/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php
index 1a68a15..74816eb 100644
--- a/core/modules/comment/src/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php
+++ b/core/modules/comment/src/Plugin/Field/FieldFormatter/CommentDefaultFormatter.php
@@ -137,7 +137,7 @@ public function __construct($plugin_id, $plugin_definition, FieldDefinitionInter
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
     $output = array();
 
diff --git a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeCustomFormatter.php b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeCustomFormatter.php
index 0537ac7..af6fd37 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeCustomFormatter.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeCustomFormatter.php
@@ -36,7 +36,7 @@ public static function defaultSettings() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php
index e78e8b1..9b2735c 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeDefaultFormatter.php
@@ -36,7 +36,7 @@ public static function defaultSettings() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimePlainFormatter.php b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimePlainFormatter.php
index 150afcc..c22445e 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimePlainFormatter.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimePlainFormatter.php
@@ -25,7 +25,7 @@ class DateTimePlainFormatter extends DateTimeFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeTimeAgoFormatter.php b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeTimeAgoFormatter.php
index edc0cac..851f59b 100644
--- a/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeTimeAgoFormatter.php
+++ b/core/modules/datetime/src/Plugin/Field/FieldFormatter/DateTimeTimeAgoFormatter.php
@@ -107,7 +107,7 @@ public static function create(ContainerInterface $container, array $configuratio
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldApplicableFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldApplicableFormatter.php
index 1d70ec2..c7bc49c 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldApplicableFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldApplicableFormatter.php
@@ -38,7 +38,7 @@ public static function isApplicable(FieldDefinitionInterface $field_definition)
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     return array('#markup' => 'Nothing to see here');
   }
 }
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldDefaultFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldDefaultFormatter.php
index ea52b40..1fa8bfb 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldDefaultFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldDefaultFormatter.php
@@ -62,7 +62,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptyFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptyFormatter.php
index 89bdfa8..cab0c17 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptyFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptyFormatter.php
@@ -35,7 +35,7 @@ public static function defaultSettings() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     if ($items->isEmpty()) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptySettingFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptySettingFormatter.php
index a9eafd4..b9bfdd7 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptySettingFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldEmptySettingFormatter.php
@@ -63,7 +63,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     if (!empty($items)) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldMultipleFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldMultipleFormatter.php
index 2cf5f2d..40b6558 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldMultipleFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldMultipleFormatter.php
@@ -63,7 +63,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     if (!empty($items)) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldNoSettingsFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldNoSettingsFormatter.php
index 3cd461e..d01ef8b 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldNoSettingsFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldNoSettingsFormatter.php
@@ -27,7 +27,7 @@ class TestFieldNoSettingsFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldPrepareViewFormatter.php b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldPrepareViewFormatter.php
index d474f39..708a404 100644
--- a/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldPrepareViewFormatter.php
+++ b/core/modules/field/tests/modules/field_test/src/Plugin/Field/FieldFormatter/TestFieldPrepareViewFormatter.php
@@ -75,7 +75,7 @@ public function prepareView(array $entities_items) {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/BaseFieldFileFormatterBase.php b/core/modules/file/src/Plugin/Field/FieldFormatter/BaseFieldFileFormatterBase.php
index 5d99b7c..fbe14f0 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/BaseFieldFileFormatterBase.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/BaseFieldFileFormatterBase.php
@@ -46,7 +46,7 @@ public function settingsForm(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     $url = NULL;
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/FileSize.php b/core/modules/file/src/Plugin/Field/FieldFormatter/FileSize.php
index 002b654..fcf74c6 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/FileSize.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/FileSize.php
@@ -34,7 +34,7 @@ public static function isApplicable(FieldDefinitionInterface $field_definition)
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
index e789236..a89d2fc 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/GenericFileFormatter.php
@@ -26,11 +26,9 @@ class GenericFileFormatter extends FileFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $file) {
       $item = $file->_referringItem;
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/RSSEnclosureFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/RSSEnclosureFormatter.php
index 9e58de2..1b0e61d 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/RSSEnclosureFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/RSSEnclosureFormatter.php
@@ -26,11 +26,9 @@ class RSSEnclosureFormatter extends FileFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $entity = $items->getEntity();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
     // Add the first file as an enclosure to the RSS item. RSS allows only one
     // enclosure per item. See: http://en.wikipedia.org/wiki/RSS_enclosure
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $file) {
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
index 8f6f773..8307973 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/TableFormatter.php
@@ -26,11 +26,9 @@ class TableFormatter extends FileFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     if ($files = $this->getEntitiesToView($items, $langcode)) {
       $header = array(t('Attachment'), t('Size'));
diff --git a/core/modules/file/src/Plugin/Field/FieldFormatter/UrlPlainFormatter.php b/core/modules/file/src/Plugin/Field/FieldFormatter/UrlPlainFormatter.php
index c1dea6c..96be320 100644
--- a/core/modules/file/src/Plugin/Field/FieldFormatter/UrlPlainFormatter.php
+++ b/core/modules/file/src/Plugin/Field/FieldFormatter/UrlPlainFormatter.php
@@ -26,11 +26,9 @@ class UrlPlainFormatter extends FileFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $file) {
       $elements[$delta] = array(
diff --git a/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php b/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php
index 6ced6a3..2e58d2e 100644
--- a/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php
+++ b/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatter.php
@@ -177,11 +177,9 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
     $files = $this->getEntitiesToView($items, $langcode);
 
     // Early opt-out if the field is empty.
diff --git a/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php b/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php
index 54cf888..de96ba3 100644
--- a/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php
+++ b/core/modules/image/src/Plugin/Field/FieldFormatter/ImageFormatterBase.php
@@ -20,7 +20,7 @@
   /**
    * {@inheritdoc}
    */
-  protected function getEntitiesToView(EntityReferenceFieldItemListInterface $items, $langcode = NULL) {
+  protected function getEntitiesToView(EntityReferenceFieldItemListInterface $items, $langcode) {
     // Add the default image if needed.
     if ($items->isEmpty()) {
       $default_image = $this->getFieldSetting('default_image');
@@ -29,9 +29,7 @@ protected function getEntitiesToView(EntityReferenceFieldItemListInterface $item
       if (empty($default_image['uuid']) && $this->fieldDefinition instanceof FieldConfigInterface) {
         $default_image = $this->fieldDefinition->getFieldStorageDefinition()->getSetting('default_image');
       }
-      if (empty($langcode)) {
-        $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-      }
+      $this->setMissingLangcode($langcode);
 
       if (!empty($default_image['uuid']) && $file = \Drupal::entityManager()->loadEntityByUuid('file', $default_image['uuid'])) {
         // Clone the FieldItemList into a runtime-only object for the formatter,
diff --git a/core/modules/link/src/Plugin/Field/FieldFormatter/LinkFormatter.php b/core/modules/link/src/Plugin/Field/FieldFormatter/LinkFormatter.php
index 21309c8..d978058 100644
--- a/core/modules/link/src/Plugin/Field/FieldFormatter/LinkFormatter.php
+++ b/core/modules/link/src/Plugin/Field/FieldFormatter/LinkFormatter.php
@@ -176,7 +176,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $element = array();
     $entity = $items->getEntity();
     $settings = $this->getSettings();
diff --git a/core/modules/link/src/Plugin/Field/FieldFormatter/LinkSeparateFormatter.php b/core/modules/link/src/Plugin/Field/FieldFormatter/LinkSeparateFormatter.php
index 3d00338..5e3f458 100644
--- a/core/modules/link/src/Plugin/Field/FieldFormatter/LinkSeparateFormatter.php
+++ b/core/modules/link/src/Plugin/Field/FieldFormatter/LinkSeparateFormatter.php
@@ -42,7 +42,7 @@ public static function defaultSettings() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $element = array();
     $entity = $items->getEntity();
     $settings = $this->getSettings();
diff --git a/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsDefaultFormatter.php b/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsDefaultFormatter.php
index 1d7f066..81b299f 100644
--- a/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsDefaultFormatter.php
+++ b/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsDefaultFormatter.php
@@ -33,7 +33,7 @@ class OptionsDefaultFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     // Only collect allowed options if there are actually items to display.
diff --git a/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsKeyFormatter.php b/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsKeyFormatter.php
index 370abfc..138d709 100644
--- a/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsKeyFormatter.php
+++ b/core/modules/options/src/Plugin/Field/FieldFormatter/OptionsKeyFormatter.php
@@ -32,7 +32,7 @@ class OptionsKeyFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
index 47d19d2..d1290cb 100644
--- a/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
+++ b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
@@ -192,9 +192,9 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    $files = $this->getEntitiesToView($items);
+    $files = $this->getEntitiesToView($items, $langcode);
 
     // Early opt-out if the field is empty.
     if (empty($files)) {
diff --git a/core/modules/responsive_image/tests/modules/responsive_image_test_module/src/Plugin/Field/FieldFormatter/ResponsiveImageTestFormatter.php b/core/modules/responsive_image/tests/modules/responsive_image_test_module/src/Plugin/Field/FieldFormatter/ResponsiveImageTestFormatter.php
index ef11c05..ac9644c 100644
--- a/core/modules/responsive_image/tests/modules/responsive_image_test_module/src/Plugin/Field/FieldFormatter/ResponsiveImageTestFormatter.php
+++ b/core/modules/responsive_image/tests/modules/responsive_image_test_module/src/Plugin/Field/FieldFormatter/ResponsiveImageTestFormatter.php
@@ -26,8 +26,8 @@ class ResponsiveImageTestFormatter extends ResponsiveImageFormatter {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
-    $elements = parent::viewElements($items);
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = parent::viewElements($items, $langcode);
     // Unset #item_attributes to test that the theme function can handle that.
     foreach ($elements as &$element) {
       if (isset($element['#item_attributes'])) {
diff --git a/core/modules/taxonomy/src/Plugin/Field/FieldFormatter/EntityReferenceTaxonomyTermRssFormatter.php b/core/modules/taxonomy/src/Plugin/Field/FieldFormatter/EntityReferenceTaxonomyTermRssFormatter.php
index 6afb331..826523a 100644
--- a/core/modules/taxonomy/src/Plugin/Field/FieldFormatter/EntityReferenceTaxonomyTermRssFormatter.php
+++ b/core/modules/taxonomy/src/Plugin/Field/FieldFormatter/EntityReferenceTaxonomyTermRssFormatter.php
@@ -29,12 +29,10 @@ class EntityReferenceTaxonomyTermRssFormatter extends EntityReferenceFormatterBa
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $parent_entity = $items->getEntity();
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
       $parent_entity->rss_elements[] = array(
diff --git a/core/modules/telephone/src/Plugin/Field/FieldFormatter/TelephoneLinkFormatter.php b/core/modules/telephone/src/Plugin/Field/FieldFormatter/TelephoneLinkFormatter.php
index dab07cd..8b3749b 100644
--- a/core/modules/telephone/src/Plugin/Field/FieldFormatter/TelephoneLinkFormatter.php
+++ b/core/modules/telephone/src/Plugin/Field/FieldFormatter/TelephoneLinkFormatter.php
@@ -67,7 +67,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $element = array();
     $title_setting = $this->getSetting('title');
 
diff --git a/core/modules/text/src/Plugin/Field/FieldFormatter/TextDefaultFormatter.php b/core/modules/text/src/Plugin/Field/FieldFormatter/TextDefaultFormatter.php
index 4e3aa69..2990c99 100644
--- a/core/modules/text/src/Plugin/Field/FieldFormatter/TextDefaultFormatter.php
+++ b/core/modules/text/src/Plugin/Field/FieldFormatter/TextDefaultFormatter.php
@@ -31,7 +31,7 @@ class TextDefaultFormatter extends FormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     // The ProcessedText element already handles cache context & tag bubbling.
diff --git a/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
index 302ce55..9cb3038 100644
--- a/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
+++ b/core/modules/text/src/Plugin/Field/FieldFormatter/TextTrimmedFormatter.php
@@ -70,7 +70,7 @@ public function settingsSummary() {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
 
     $render_as_summary = function (&$element) {
diff --git a/core/modules/user/src/Plugin/Field/FieldFormatter/AuthorFormatter.php b/core/modules/user/src/Plugin/Field/FieldFormatter/AuthorFormatter.php
index 122e03d..7a6ef1b 100644
--- a/core/modules/user/src/Plugin/Field/FieldFormatter/AuthorFormatter.php
+++ b/core/modules/user/src/Plugin/Field/FieldFormatter/AuthorFormatter.php
@@ -31,11 +31,9 @@ class AuthorFormatter extends EntityReferenceFormatterBase {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = array();
-    if (empty($langcode)) {
-      $langcode = \Drupal::languageManager()->getCurrentLanguage(LanguageInterface::TYPE_CONTENT)->getId();
-    }
+    $this->setMissingLangcode($langcode);
 
     foreach ($this->getEntitiesToView($items, $langcode) as $delta => $entity) {
       /** @var $referenced_user \Drupal\user\UserInterface */
diff --git a/core/modules/user/src/Plugin/Field/FieldFormatter/UserNameFormatter.php b/core/modules/user/src/Plugin/Field/FieldFormatter/UserNameFormatter.php
index 083da3c..bfa8ab5 100644
--- a/core/modules/user/src/Plugin/Field/FieldFormatter/UserNameFormatter.php
+++ b/core/modules/user/src/Plugin/Field/FieldFormatter/UserNameFormatter.php
@@ -54,7 +54,7 @@ public function settingsForm(array $form, FormStateInterface $form_state) {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
+  public function viewElements(FieldItemListInterface $items, $langcode) {
     $elements = [];
 
     foreach ($items as $delta => $item) {
diff --git a/core/modules/views/tests/modules/views_test_formatter/src/Plugin/Field/FieldFormatter/AttachmentTestFormatter.php b/core/modules/views/tests/modules/views_test_formatter/src/Plugin/Field/FieldFormatter/AttachmentTestFormatter.php
index e96f6b3..86600c6 100644
--- a/core/modules/views/tests/modules/views_test_formatter/src/Plugin/Field/FieldFormatter/AttachmentTestFormatter.php
+++ b/core/modules/views/tests/modules/views_test_formatter/src/Plugin/Field/FieldFormatter/AttachmentTestFormatter.php
@@ -28,8 +28,8 @@ class AttachmentTestFormatter extends NumericUnformattedFormatter {
   /**
    * {@inheritdoc}
    */
-  public function viewElements(FieldItemListInterface $items, $langcode = NULL) {
-    $elements = parent::viewElements($items);
+  public function viewElements(FieldItemListInterface $items, $langcode) {
+    $elements = parent::viewElements($items, $langcode);
 
     // Add dummy attachments.
     $entity_id = $items->getEntity()->id();