diff --git a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
index a988135..9d9cfd7 100644
--- a/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
+++ b/core/lib/Drupal/Core/Config/Entity/ConfigEntityBase.php
@@ -8,8 +8,10 @@
 namespace Drupal\Core\Config\Entity;
 
 use Drupal\Core\Entity\Entity;
+use Drupal\Core\Entity\EntityMalformedException;
 use Drupal\Core\Entity\EntityStorageControllerInterface;
 use Drupal\Core\Config\ConfigDuplicateUUIDException;
+use Drupal\Core\Entity\EntityTypeMalformedException;
 
 /**
  * Defines a base configuration entity class.
@@ -48,6 +50,13 @@
   public function __construct(array $values, $entity_type) {
     parent::__construct($values, $entity_type);
 
+    // The configuration translation system relies on the 'langcode' key to
+    // perform language detection and overrides. Therefore it is not valid to
+    // set the language entity key to anything other than 'langcode'.
+    if ($this->entityInfo()->getKey('langcode') != 'langcode') {
+      throw new EntityTypeMalformedException("The entity type '$entity_type' has an invalid 'langcode' entity key.");
+    }
+
     // Backup the original ID, if any.
     // Configuration entity IDs are strings, and '0' is a valid ID.
     $original_id = $this->id();
diff --git a/core/lib/Drupal/Core/Entity/EntityManager.php b/core/lib/Drupal/Core/Entity/EntityManager.php
index 6221633..80814cc 100644
--- a/core/lib/Drupal/Core/Entity/EntityManager.php
+++ b/core/lib/Drupal/Core/Entity/EntityManager.php
@@ -353,11 +353,12 @@ public function getFieldDefinitions($entity_type, $bundle = NULL) {
         $this->moduleHandler->alter($hooks, $this->entityFieldInfo[$entity_type], $entity_type);
 
         // Ensure all basic fields are not defined as translatable.
-        $keys = array_intersect_key(array_filter($entity_info->getKeys()), array_flip(array('id', 'revision', 'uuid', 'bundle')));
-        $untranslatable_fields = array_flip(array('langcode') + $keys);
+        // @todo I don't think this works as expected. I think it should
+        //   array_flip additionally.
+        $untranslatable_fields = array_filter($entity_info->getKeys());
         foreach (array('definitions', 'optional') as $key) {
           foreach ($this->entityFieldInfo[$entity_type][$key] as $field_name => &$definition) {
-            if (isset($untranslatable_fields[$field_name]) && $definition->isTranslatable()) {
+            if (in_array($field_name, $untranslatable_fields) && $definition->isTranslatable()) {
               throw new \LogicException(String::format('The @field field cannot be translatable.', array('@field' => $definition->getLabel())));
             }
           }
diff --git a/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
index 9edc094..b784952 100644
--- a/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -236,7 +236,11 @@ public function isFieldDataCacheable() {
    * {@inheritdoc}
    */
   public function getKeys() {
-    return $this->entity_keys + array('revision' => '', 'bundle' => '', 'langcode' => 'langcode');
+    return $this->entity_keys + array(
+      'revision' => '',
+      'bundle' => '',
+      'langcode' => 'langcode',
+    );
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Entity/EntityTypeMalformedException.php b/core/lib/Drupal/Core/Entity/EntityTypeMalformedException.php
new file mode 100644
index 0000000..f1f7cb3
--- /dev/null
+++ b/core/lib/Drupal/Core/Entity/EntityTypeMalformedException.php
@@ -0,0 +1,13 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\Core\Entity\EntityTypeMalformedException.
+ */
+
+namespace Drupal\Core\Entity;
+
+/**
+ * Defines an exception thrown when a malformed entity is passed.
+ */
+class EntityTypeMalformedException extends \Exception { }
diff --git a/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php b/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php
index 282987d..fac167f 100644
--- a/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php
+++ b/core/lib/Drupal/Core/Entity/FieldableDatabaseStorageController.php
@@ -284,7 +284,8 @@ protected function attachPropertyData(array &$entities) {
 
         // Field values in default language are stored with
         // Language::LANGCODE_DEFAULT as key.
-        $langcode = empty($values['default_langcode']) ? $values['langcode'] : Language::LANGCODE_DEFAULT;
+        $language_key = 'langcode';
+        $langcode = empty($values["default_$language_key"]) ? $values[$language_key] : Language::LANGCODE_DEFAULT;
         $translations[$id][$langcode] = TRUE;
 
         foreach (array_keys($field_definitions) as $field_name) {
@@ -362,13 +363,14 @@ protected function buildPropertyQuery(QueryInterface $entity_query, array $value
       //   apply to the default language. See http://drupal.org/node/1866330.
       // Default to the original entity language if not explicitly specified
       // otherwise.
-      if (!array_key_exists('default_langcode', $values)) {
-        $values['default_langcode'] = 1;
+      $default_langcode_key = "default_{$this->langcode_key}";
+      if (!array_key_exists($default_langcode_key, $values)) {
+        $values[$default_langcode_key] = 1;
       }
       // If the 'default_langcode' flag is explicitly not set, we do not care
       // whether the queried values are in the original entity language or not.
-      elseif ($values['default_langcode'] === NULL) {
-        unset($values['default_langcode']);
+      elseif ($values[$default_langcode_key] === NULL) {
+        unset($values[$default_langcode_key]);
       }
     }