Set demo user back to English at login.

app/Handlers/Events/UserEventHandler.php

@@ -108,6 +108,27 @@ class UserEventHandler
         return true;
     }
 
+    /**
+     * @param Login $event
+     *
+     * @return bool
+     */
+    function demoUserBackToEnglish(Login $event): bool
+    {
+        /** @var UserRepositoryInterface $repository */
+        $repository = app(UserRepositoryInterface::class);
+
+        /** @var User $user */
+        $user = $event->user;
+        if ($repository->hasRole($user, 'demo')) {
+            // set user back to English.
+            app('preferences')->setForUser($user, 'language', 'en_US');
+            app('preferences')->mark();
+        }
+
+        return true;
+    }
+
     /**
      * @param UserChangedEmail $event
      *

app/Providers/EventServiceProvider.php

@@ -64,6 +64,7 @@ class EventServiceProvider extends ServiceProvider
             // is a User related event.
             Login::class                       => [
                 'FireflyIII\Handlers\Events\UserEventHandler@checkSingleUserIsAdmin',
+                'FireflyIII\Handlers\Events\UserEventHandler@demoUserBackToEnglish',
 
             ],
             RequestedVersionCheckStatus::class => [