core/modules/ckeditor/ckeditor.module              | 11 +--
 .../modules/ckeditor/src/CKEditorPluginManager.php | 30 +++++--
 .../tests/src/Unit/CKEditorPluginManagerTest.php   | 97 ++++++++++++++++++++++
 3 files changed, 124 insertions(+), 14 deletions(-)

diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index d39d95a..b7ba596 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -70,11 +70,12 @@ function ckeditor_ckeditor_css_alter(array &$css, Editor $editor) {
     $css[] = drupal_get_path('module', 'ckeditor') . '/css/plugins/drupalimagecaption/ckeditor.drupalimagecaption.css';
   }
 
-  // @todo: This should only be loaded when the Language button is enabled;
-  //    the CKEditor text editor plugin should load this automatically,
-  //    CKEditor plugins that want to load additional CSS *IN* CKEditor
-  //    shouldn't have to implement hook_ckeditor_css_alter().
-  $css[] = drupal_get_path('module', 'ckeditor') . '/css/plugins/language/ckeditor.language.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';
+  }
 }
 
 /**
diff --git a/core/modules/ckeditor/src/CKEditorPluginManager.php b/core/modules/ckeditor/src/CKEditorPluginManager.php
index 07d811f..cd0a1bf 100644
--- a/core/modules/ckeditor/src/CKEditorPluginManager.php
+++ b/core/modules/ckeditor/src/CKEditorPluginManager.php
@@ -71,15 +71,7 @@ public function __construct(\Traversable $namespaces, CacheBackendInterface $cac
    */
   public function getEnabledPluginFiles(Editor $editor, $include_internal_plugins = FALSE) {
     $plugins = array_keys($this->getDefinitions());
-    // Flatten each row.
-    $toolbar_rows = array();
-    $settings = $editor->getSettings();
-    foreach ($settings['toolbar']['rows'] as $row_number => $row) {
-      $toolbar_rows[] = array_reduce($settings['toolbar']['rows'][$row_number], function (&$result, $button_group) {
-        return array_merge($result, $button_group['items']);
-      }, array());
-    }
-    $toolbar_buttons = array_unique(NestedArray::mergeDeepArray($toolbar_rows));
+    $toolbar_buttons = $this->getEnabledButtons($editor);
     $enabled_plugins = array();
     $additional_plugins = array();
 
@@ -121,6 +113,26 @@ public function getEnabledPluginFiles(Editor $editor, $include_internal_plugins
   }
 
   /**
+   * Gets the enabled toolbar buttons in the given text editor instance.
+   *
+   * @param \Drupal\editor\Entity\Editor $editor
+   *   A configured text editor object.
+   *
+   * @return string[]
+   *   A list of the toolbar buttons enabled in the given text editor instance.
+   */
+  public static function getEnabledButtons(Editor $editor) {
+    $toolbar_rows = [];
+    $settings = $editor->getSettings();
+    foreach ($settings['toolbar']['rows'] as $row_number => $row) {
+      $toolbar_rows[] = array_reduce($settings['toolbar']['rows'][$row_number], function (&$result, $button_group) {
+        return array_merge($result, $button_group['items']);
+      }, []);
+    }
+    return array_unique(NestedArray::mergeDeepArray($toolbar_rows));
+  }
+
+  /**
    * Retrieves all available CKEditor buttons, keyed by plugin ID.
    *
    * @return array
diff --git a/core/modules/ckeditor/tests/src/Unit/CKEditorPluginManagerTest.php b/core/modules/ckeditor/tests/src/Unit/CKEditorPluginManagerTest.php
index e69de29..4184347 100644
--- a/core/modules/ckeditor/tests/src/Unit/CKEditorPluginManagerTest.php
+++ b/core/modules/ckeditor/tests/src/Unit/CKEditorPluginManagerTest.php
@@ -0,0 +1,97 @@
+<?php
+
+namespace Drupal\Tests\ckeditor\Unit;
+
+use Drupal\ckeditor\CKEditorPluginManager;
+use Drupal\editor\Entity\Editor;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @coversDefaultClass \Drupal\ckeditor\CKEditorPluginManager
+ *
+ * @group ckeditor
+ */
+class CKEditorPluginManagerTest extends UnitTestCase {
+
+  /**
+   * Provides a list of configs to test.
+   */
+  public function providerGetEnabledButtons() {
+    return [
+      'empty' => [
+        [],
+        []
+      ],
+      '1 row, 1 group' => [
+        [
+          // Row 1.
+          [
+            // Group 1.
+            ['name' => 'Formatting', 'items' => ['Bold', 'Italic']],
+          ]
+        ],
+        ['Bold', 'Italic']
+      ],
+      '1 row, >1 groups' => [
+        [
+          // Row 1.
+          [
+            // Group 1.
+            ['name' => 'Formatting', 'items' => ['Bold', 'Italic']],
+            // Group 2.
+            ['name' => 'Linking', 'items' => ['Link']],
+          ],
+        ],
+        ['Bold', 'Italic', 'Link']
+      ],
+      '2 rows, 1 group each' => [
+        [
+          // Row 1.
+          [
+            // Group 1.
+            ['name' => 'Formatting', 'items' => ['Bold', 'Italic']],
+          ],
+          // Row 2.
+          [
+            // Group 1.
+            ['name' => 'Tools', 'items' => ['Source']],
+          ],
+        ],
+        ['Bold', 'Italic', 'Source'],
+      ],
+      '2 rows, >1 groups each' => [
+        [
+          // Row 1.
+          [
+            // Group 1.
+            ['name' => 'Formatting', 'items' => ['Bold', 'Italic']],
+            // Group 2.
+            ['name' => 'Linking', 'items' => ['Link']],
+        ],
+          // Row 2.
+          [
+            // Group 1.
+            ['name' => 'Tools', 'items' => ['Source']],
+            // Group 2.
+            ['name' => 'Advanced', 'items' => ['Llama']],
+          ],
+        ],
+        ['Bold', 'Italic', 'Link', 'Source', 'Llama']
+      ],
+    ];
+  }
+
+  /**
+   * @covers ::getEnabledButtons
+   * @dataProvider providerGetEnabledButtons
+   */
+  public function testGetEnabledButtons(array $toolbar_rows, array $expected_buttons) {
+    $editor= $this->prophesize(Editor::class);
+    $editor->getSettings()
+      ->willReturn(['toolbar' => ['rows' => $toolbar_rows]]);
+
+    $enabled_buttons = CKEditorPluginManager::getEnabledButtons($editor->reveal());
+    $this->assertEquals($expected_buttons, $enabled_buttons);
+  }
+
+}