FIX: Add missing translation. (#7014)

config/locales/client.en.yml

@@ -1550,6 +1550,7 @@ en:
         posted: '{{username}} posted in "{{topic}}" - {{site_title}}'
         private_message: '{{username}} sent you a personal message in "{{topic}}" - {{site_title}}'
         linked: '{{username}} linked to your post from "{{topic}}" - {{site_title}}'
+        watching_first_post: '{{username}} created a new topic "{{topic}}" - {{site_title}}'
         confirm_title: "Notifications enabled - %{site_title}"
         confirm_body: "Success! Notifications have been enabled."