Issue #2224691 by Sweetchuck: JavaScript Drupal.t and Drupal.formatPlural only works for non-english languages.

core/includes/common.inc

@@ -2073,7 +2073,12 @@ function _drupal_add_js($data = NULL, $options = NULL) {
             ksort($current_query);
             $path['currentQuery'] = (object) $current_query;
           }
-          $javascript['settings']['data'][] = array('path' => $path);
+          $javascript['settings']['data'][] = array(
+            'path' => $path,
+            'locale' => array(
+              'pluralDelimiter' => LOCALE_PLURAL_DELIMITER,
+            ),
+          );
         }
         // All JavaScript settings are placed in the header of the page with
         // the library weight so that inline scripts appear afterwards.

core/misc/drupal.js

@@ -364,7 +364,7 @@ if (window.jQuery) {
     args = args || {};
     args['@count'] = count;
 
-    var pluralDelimiter = Drupal.locale.pluralDelimiter;
+    var pluralDelimiter = drupalSettings.locale.pluralDelimiter;
 
     // Determine the index of the plural form.
     var index = Drupal.locale.pluralFormula ? Drupal.locale.pluralFormula(args['@count']) : ((args['@count'] === 1) ? 0 : 1);

core/modules/locale/locale.module

@@ -1296,9 +1296,7 @@ function _locale_rebuild_js($langcode = NULL) {
   $data_hash = NULL;
   $data = $status = '';
   if (!empty($translations)) {
-    $data = array(
-      'pluralDelimiter: ' . Json::encode(LOCALE_PLURAL_DELIMITER),
-    );
+    $data = array();
 
     $locale_plurals = \Drupal::state()->get('locale.translation.plurals') ?: array();
     if (!empty($locale_plurals[$language->id]['formula'])) {