Hello. This is not in the language file at this time, but you CAN change it. Go to the file:
/components/com_dtregister/dtregister.php
Search for: \"is required\" and you\'ll find this:
{$label} is required
or
$label is required
in multiple places. The $label pulls in the field name. Leave that part as is, but you can change the rest of the text there. Be sure to back up your file prior to making changes in case you modify something you shouldn\'t