add missing translation for email_error_isemail

Author: Patrick Anders

interface/web/mail/lib/lang/de_mail_forward.lng

@@ -6,4 +6,5 @@ $wb['limit_mailforward_txt'] = 'Die maximale Anzahl an E-Mail Weiterleitungen f
 $wb['duplicate_mailbox_txt'] = 'Es existiert bereits ein E-Mail Konto mit dieser Adresse.';
 $wb['domain_txt'] = 'Domain';
 $wb['source_txt'] = 'Quell E-Mail Adresse';
+$wb['email_error_isemail'] = 'Bitte geben Sie eine gültige E-Mail Adresse an.';
 ?>

interface/web/mail/lib/lang/en_mail_forward.lng

@@ -6,4 +6,5 @@ $wb["limit_mailforward_txt"] = 'The max. number of email forwarders for your acc
 $wb["duplicate_mailbox_txt"] = 'There is already a mailbox with this email address';
 $wb['domain_txt'] = 'Domain';
 $wb["source_txt"] = 'Source Email';
+$wb['email_error_isemail'] = 'Please enter a valid email address.';
 ?>