Add 'is-active' class to language block links of current language

Closes #2775651

core/modules/language/src/Plugin/Block/LanguageBlock.php

@@ -100,8 +100,13 @@ public function build() {
       $url = Url::fromRouteMatch($route_match);
     }
     $links = $this->languageManager->getLanguageSwitchLinks($type, $url);
+    $current_language_code = $this->languageManager->getCurrentLanguage()->getId();
 
     if (isset($links->links)) {
+      // Add the active state.
+      if (isset($links->links[$current_language_code])) {
+        $links->links[$current_language_code]['attributes']['class'][] = 'is-active';
+      }
       $build = [
         '#theme' => 'links__language_block',
         '#links' => $links->links,