If you want to allow your user's to signup with arabic username, Then please code this code and paste is your child theme functions.php file.
function classiera_allow_arabic_username($username, $raw_username, $strict) {
$username = wp_strip_all_tags( $raw_username );
$username = remove_accents( $username );
$username = preg_replace( '|%([a-fA-F0-9][a-fA-F0-9])|', '', $username );
$username = preg_replace( '/&.+?;/', '', $username ); // Kill entities
// If strict, reduce to ASCII and Arabic characters for max portability.
if ( $strict )
$username = preg_replace( '|[^a-z\p{Arabic}0-9 _.\-@]|iu', '', $username );
$username = trim( $username );
// Consolidate contiguous whitespace
$username = preg_replace( '|\s+|', ' ', $username );
return $username;
}
add_filter('sanitize_user', 'classiera_allow_arabic_username', 10, 3);