whmcsExistingDomains/lang/english.php

<?php
$_ADDONLANG['UseaDomainIhaveAlreadyPurchased'] = "Use a domain I have already purchased";
$_ADDONLANG['UseaSubDomainIhaveAlreadyPurchased'] = "Use a sub domain of domain I have already purchased";
$_ADDONLANG['UseaSubDomainTooltip'] = "Please enter your sub domain";
$_ADDONLANG['errordomain'] = " not acceptable because already in use";