Add translation for German GND

backend/pl/event.json

@@ -90,6 +90,7 @@
         "similar_event_already_added": "A similar event had already been added",
         "event_added": "The event has been added",
         "edit_event": "Edit the event",
-        "clone_event": "Clone the event"
+        "clone_event": "Clone the event",
+        "gnd": "Gemeinsame Normdatei"
     }
 }