Issue #2936793 follow-up by tim.plunkett: Fix PHP5's hacking and wheezing.

(cherry picked from commit 02af9bc3)

core/lib/Drupal/Core/Field/Plugin/Field/FieldType/LanguageItem.php

@@ -125,7 +125,7 @@ public static function generateSampleValue(FieldDefinitionInterface $field_defin
     // Defer to the callback in the item definition as it can be overridden.
     $constraint = $field_definition->getItemDefinition()->getConstraint('ComplexData');
     if (isset($constraint['value']['AllowedValues']['callback'])) {
-      $languages = $constraint['value']['AllowedValues']['callback']();
+      $languages = call_user_func($constraint['value']['AllowedValues']['callback']);
     }
     else {
       $languages = static::getAllowedLanguageCodes();