Issue #2681845 by thpoul: Use CKEditorPluginCssInterface to add the CKEditor language plugin css

(cherry picked from commit d62ca100)

core/modules/ckeditor/ckeditor.module

@@ -64,13 +64,6 @@ function ckeditor_ckeditor_css_alter(array &$css, Editor $editor) {
   if ($editor->getFilterFormat()->filters('filter_caption')->status) {
     $css[] = drupal_get_path('module', 'filter') . '/css/filter.caption.css';
   }
-
-  // @todo: Remove in https://www.drupal.org/node/2645100.
-  /** @var \Drupal\ckeditor\CKEditorPluginManager $ckeditor_plugin_manager */
-  $ckeditor_plugin_manager = \Drupal::service('plugin.manager.ckeditor.plugin');
-  if (in_array('Language', $ckeditor_plugin_manager->getEnabledButtons($editor))) {
-    $css[] = drupal_get_path('module', 'ckeditor') . '/css/plugins/language/ckeditor.language.css';
-  }
 }
 
 /**

core/modules/ckeditor/src/Plugin/CKEditorPlugin/Language.php

@@ -9,6 +9,7 @@
 
 use Drupal\ckeditor\CKEditorPluginBase;
 use Drupal\ckeditor\CKEditorPluginConfigurableInterface;
+use Drupal\ckeditor\CKEditorPluginCssInterface;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Language\LanguageManager;
 use Drupal\Core\Language\LanguageInterface;
@@ -23,7 +24,7 @@
  *   label = @Translation("Language")
  * )
  */
-class Language extends CKEditorPluginBase implements CKEditorPluginConfigurableInterface {
+class Language extends CKEditorPluginBase implements CKEditorPluginConfigurableInterface, CKEditorPluginCssInterface {
 
   /**
    * {@inheritdoc}
@@ -128,4 +129,13 @@ public function settingsForm(array $form, FormStateInterface $form_state, Editor
     return $form;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  function getCssFiles(Editor $editor) {
+    return array(
+        drupal_get_path('module', 'ckeditor') . '/css/plugins/language/ckeditor.language.css'
+    );
+  }
+
 }