You can use Strings.STS to override message. Just add this code to your page or master page if you want to apply all your site.
<script type="text/javascript">
$(document).ready(function () {
try {
if (Strings.STS.L_SPClientRequiredValidatorError != null) {
Strings.STS.L_SPClientRequiredValidatorError = "What ever you want!";
}
}
catch (exception) { }
});
</script>
For more information:
SharePoint 2010 use message:
"You must specify a value for this required field." It showed after validating from the server.
SharePoint
2013 shows a message:
"You can't leave this blank."
after validating from the client. But, if you use SharePoint webpart and SharePoint control, it still shows a message same as SharePoint 2010. It makes it inconsistent from your site.