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);