Add translation variables for making the themator translatable

themator/en/backend.json

@@ -165,6 +165,7 @@
         "source_list_not_listed": "* Here you can add sources (= references to literature, that is not yet online) or link already known references.<br \/>\r\nShould the source not be in the list, please click",
         "mandatory_title_source": "You need to add a title for the souce!",
         "linkdisplayname": "Link Display Name",
-        "link": "Link"
+        "link": "Link",
+        "translation_language": "Language"
     }
 }