Waitlist button not translatable

I’ve used this plugin for a couple of years, but have only now activated the waitlist option. The button to reserve on the waitlist shows: ‘Join Waitlist’.

I can’t find where or how this text can be translated. Even with Loco the text can’t be found or changed. There is this (hard) coded? I need to get is translated now.

The page I need help with: [log in to see the link]