It is a known fact that per default WordPress does not allow to use special characters in usernames. Non-latin characters are silently filtered out and your users cannot create accounts containing cyrillic (russian) or arabic letters. You can find many info on this online – example: https://wordpress.org/support/topic/problems-with-allowing-non-latin-characters-in-usernames
You can test by creating an username from admin – add new with username in a non latin language and you’ll see wordpress will now allow you to do that. It doesn’t matter what theme you test with. Being a general WordPress issue, it happens with any theme.
This is a screenshot as example:
Because of this limit, when using WP RENTALS theme front end register form, google login, facebook login or yahoo login – if the username is in non latin – the username will not be created. Please understand this this not a theme error.
From online resources there is this url https://wordpress.org/support/topic/username-in-hebrew to modify wordpress and allow hebrew usernames. It could be tested for other non latin languages.