diff --git a/core/includes/language.inc b/core/includes/language.inc
index 2bfdc31..43e837e 100644
--- a/core/includes/language.inc
+++ b/core/includes/language.inc
@@ -13,6 +13,11 @@
 const LANGUAGE_NEGOTIATION_SELECTED = 'language-selected';
 
 /**
+ * The language is determined using the current interface language.
+ */
+const LANGUAGE_NEGOTIATION_INTERFACE = 'language-interface';
+
+/**
  * @defgroup language_negotiation Language Negotiation API functionality
  * @{
  * Functions to customize the language types and the negotiation process.
@@ -170,7 +175,10 @@ function language_types_info() {
  */
 function language_types_get_configurable() {
   $configurable = &drupal_static(__FUNCTION__);
-  return isset($configurable) ? $configurable : array_keys(array_filter(config('system.language.types')->get('configurable')));
+  if (!isset($configurable)) {
+    $configurable = array_keys(array_filter(config('system.language.types')->get('configurable')));
+  }
+  return $configurable;
 }
 
 /**
@@ -202,7 +210,6 @@ function language_types_disable($types) {
  *   configuration.
  */
 function language_types_set($configurable) {
-  include_once DRUPAL_ROOT . '/' . drupal_get_path('module', 'language') . '/language.negotiation.inc';
   // Ensure that we are getting the defined language negotiation information. An
   // invocation of module_enable() or module_disable() could outdate the cached
   // information.
@@ -212,12 +219,12 @@ function language_types_set($configurable) {
   $language_types = array();
   $negotiation_info = language_negotiation_info();
   foreach (language_types_info() as $type => $info) {
-    // Check if the language is locked. Configurability of a locked language
-    // type cannot be change using the UI.
+    // Check if the language is locked. The configuration of a locked language
+    // type cannot be changed using the UI.
     if ($locked = !empty($info['locked'])) {
       // If a locked language has default settings (by using the 'fixed'
-      // property) it will be always configurable. If it doesn't have any
-      // default setting, then it won't be configurable.
+      // property) it will be always configurable. If it has no default
+      // settings, then it won't be configurable.
       $configurable[$type] = empty($info['fixed']);
       $language_types[$type] = !$locked;
       if (!$configurable[$type]) {
@@ -231,7 +238,7 @@ function language_types_set($configurable) {
       }
     }
     else {
-      // Configurability of an unlocked language will be based on the
+      // The configuration of an unlocked language will be based on the
       // $configurable array (stored in configuration). These values may be
       // altered using the UI.
       $language_types[$type] = !empty($configurable[$type]);
diff --git a/core/modules/language/language.negotiation.inc b/core/modules/language/language.negotiation.inc
index ca41c78..4e29e47 100644
--- a/core/modules/language/language.negotiation.inc
+++ b/core/modules/language/language.negotiation.inc
@@ -18,11 +18,6 @@
 const LANGUAGE_NEGOTIATION_BROWSER = 'language-browser';
 
 /**
- * The language is determined using the current interface language.
- */
-const LANGUAGE_NEGOTIATION_INTERFACE = 'language-interface';
-
-/**
  * If no URL language, language is determined using an already detected one.
  */
 const LANGUAGE_NEGOTIATION_URL_FALLBACK = 'language-url-fallback';