Show a warning if the chosen locale by the user is not available on the system

refs #11820

application/forms/PreferenceForm.php

@@ -156,6 +156,16 @@ class PreferenceForm extends Form
      */
     public function createElements(array $formData)
     {
+        if (setlocale(LC_ALL, 0) === 'C') {
+            $this->warning(
+                $this->translate(
+                    'Your language setting is not applied because your platform is missing the corresponding locale.'
+                    . ' Make sure to install the correct language pack and restart your web server afterwards.'
+                ),
+                false
+            );
+        }
+
         if (! (bool) Config::app()->get('themes', 'disabled', false)) {
             $themes = Icinga::app()->getThemes();
             if (count($themes) > 1) {