.../plugins/language/ckeditor.language.css         | 28 ++++++++++++++++++++++
 core/themes/stable/stable.info.yml                 |  4 ++++
 2 files changed, 32 insertions(+)

diff --git a/core/themes/stable/css/ckeditor/plugins/language/ckeditor.language.css b/core/themes/stable/css/ckeditor/plugins/language/ckeditor.language.css
new file mode 100644
index 0000000..1b69284
--- /dev/null
+++ b/core/themes/stable/css/ckeditor/plugins/language/ckeditor.language.css
@@ -0,0 +1,28 @@
+/**
+ * @file
+ * Language: add styling for elements that have a language attribute.
+ */
+
+/**
+ * Show the user that a 'lang' tag has been applied by adding a thin dotted
+ * border. We also append the value of the tag between brackets, eg: '(en)'.
+ */
+[lang] {
+  outline: 1px dotted gray;
+}
+[lang]:after {
+  content: " ("attr(lang)")";
+  font-size: 10px;
+  color: #666;
+}
+
+/**
+ * The HTML element has a default 'lang' attribute.
+ * We don't want styling there.
+ */
+html[lang] {
+  outline: none;
+}
+html[lang]:after {
+  content: none;
+}
diff --git a/core/themes/stable/stable.info.yml b/core/themes/stable/stable.info.yml
index e1981fe..3476833 100644
--- a/core/themes/stable/stable.info.yml
+++ b/core/themes/stable/stable.info.yml
@@ -21,6 +21,10 @@ libraries-override:
     css:
       component:
         css/plugins/drupalimagecaption/ckeditor.drupalimagecaption.css: css/ckeditor/plugins/drupalimagecaption/ckeditor.drupalimagecaption.css
+  ckeditor/drupal.ckeditor.plugins.language:
+    css:
+      component:
+        css/plugins/language/ckeditor.language.css: css/ckeditor/plugins/language/ckeditor.language.css
   ckeditor/drupal.ckeditor.admin:
     css:
       theme: