how to let the user insert his user name in Arabic

Hi I have an Arabic website
and want to customize all the forms labels, built by usersWP’s form builder and make it in Arabic

while registration also, I want to let the user fill in the user name, first name, last name in Arabic, meanwhile it’s coming to an error message that ” Error: This username is invalid because it uses illegal characters. Please enter a valid username.”

please advice

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