Add string.

en.json

@@ -29,6 +29,7 @@
         "resend": "Resend verification.",
         "cancel": "Cancel request.",
         "check_mail": "Check your mail.",
+        "check_spam": "Be sure to check spam if you can't find it.",
         "open_mail_provider": "Open {{provider}}",
         "successful_registration": "Successfully registered!",
         "error": {