That looks like a glitch on our end. The strings have the wrong text-domain assigned to them.
I’ll get those fixed for the next release. Sorry about this.
(If you want to do a quick fix for yourself, because you need it, then open wp-content/plugins/event-tickets-plus/src/admin-views/editor/fieldset/settings-capacity.php and change all the ‘event-tickets’ textdomains to ‘event-tickets-plus’.)