diff --git a/core/core.api.php b/core/core.api.php
index 4b2fe62..11fa9d6 100644
--- a/core/core.api.php
+++ b/core/core.api.php
@@ -2236,6 +2236,61 @@ function hook_validation_constraint_alter(array &$definitions) {
 }
 
 /**
+ * Compute the plural variant index.
+ *
+ * Modules are implementing this hook in order to provide a way to compute the
+ * plural variant index for a certain combination of $count and $langcode.
+ * However, only the index provided by the most weighted module returning a
+ * valid value will be used in PluralTranslatableMarkup::getPluralIndex(). The
+ * implementation should return a valid zero-based positive integer or nothing,
+ * if he wants to pass the decision to core or other modules. Returning anything
+ * else than a zero or a positive integer will have the same effect as missing
+ * the return statement. See locale_plural_variant_index() for an example of
+ * using locale_get_plural() function to retrieve the plural variant index. You
+ * can provide your own implementation, in your custom module, when you don't
+ * want to enable the Locale module or you want to override the Locale module
+ * mechanism.
+ *
+ * @param int $count
+ *   The count value for which to retrieve the plural variant index.
+ * @param string|null $langcode
+ *   (optional) The code of the language to be checked. The implementation
+ *   should decide what to do if the language is missed or is invalid. For
+ *   example the Local module implementation locale_plural_variant_index() is
+ *   calling locale_get_plural() and that defaults to the current language.
+ *
+ * @return int|mixed
+ *   The zero-based numeric index of the plural variant to use for this $count
+ *   and $langcode combination. To avoid making a decision for a certain
+ *   combination just return nothing or any value that is not a zero or a
+ *   positive integer.
+ *
+ * @see \Drupal\Core\StringTranslation\PluralTranslatableMarkup::getPluralIndex()
+ * @see locale_plural_variant_index()
+ * @see locale_get_plural()
+ */
+function hook_plural_variant_index($count, $langcode = NULL) {
+  if ($langcode == 'en') {
+    return $count == 1 ? 0 : 1;
+  }
+  // The Romanian language has 3 plural variants.
+  elseif ($langcode == 'ro') {
+    if ($count == 1) {
+      return 0;
+    }
+    if ($count == 0 || ($count % 100 > 0 && $count % 100 < 20)) {
+      return 1;
+    }
+    return 2;
+  }
+  // In this implementation we ignore all other languages. Therefore we can
+  // return nothing or any value that is not a zero or positive integer.
+  // @code return -1; @endcode or @code return FALSE; @endcode or just omitting
+  // the return statement are the same and will pass the decision to core or
+  // other modules.
+}
+
+/**
  * @} End of "addtogroup hooks".
  */
 
diff --git a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
index 89e602f..1e23df4 100644
--- a/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
+++ b/core/lib/Drupal/Core/StringTranslation/PluralTranslatableMarkup.php
@@ -145,15 +145,20 @@ public function render() {
    * @return int
    */
   protected function getPluralIndex() {
-    // We have to test both if the function and the service exist since in
-    // certain situations it is possible that locale code might be loaded but
-    // the service does not exist. For example, where the parent test site has
-    // locale installed but the child site does not.
-    // @todo Refactor in https://www.drupal.org/node/2660338 so this code does
-    // not depend on knowing that the Locale module exists.
-    if (function_exists('locale_get_plural') && \Drupal::hasService('locale.plural.formula')) {
-      return locale_get_plural($this->count, $this->getOption('langcode'));
+    $indexes = \Drupal::moduleHandler()
+      ->invokeAll('plural_variant_index', [$this->count, $this->getOption('langcode')]);
+
+    // Pickup index provided by the most weighted module returning a valid
+    // value.
+    if ($indexes) {
+      do {
+        $index = array_pop($indexes);
+        if (is_integer($index) && $index >= 0) {
+          return $index;
+        }
+      } while ($indexes);
     }
+    // Return -1 if no module has implemented valid plural formulas.
     return -1;
   }
 
diff --git a/core/modules/locale/locale.module b/core/modules/locale/locale.module
index aecad6c..d568959 100644
--- a/core/modules/locale/locale.module
+++ b/core/modules/locale/locale.module
@@ -308,6 +308,12 @@ function locale_get_plural($count, $langcode = NULL) {
   return $plural_indexes[$langcode][$count];
 }
 
+/**
+ * Implements hook_plural_variant_index().
+ */
+function locale_plural_variant_index($count, $langcode = NULL) {
+  return locale_get_plural($count, $langcode);
+}
 
 /**
  * Implements hook_modules_installed().
diff --git a/sites/default/default.settings.php b/sites/default/default.settings.php
old mode 100644
new mode 100755