Does Plesk support national characters in domain names? For example with Arabic or Russian letters.
Yes, Plesk supports national characters in domain names which is implemented with a help of Punycode converter. Enter domain name including national characters symbols while creating the domain. But domain's Document Root in Plesk > Domains > example.com > Hosting Settings should be still specified in English, as Plesk will not recognize the Document Root set in national characters.
Note: Domain name will be transformed into English symbols according to Punycode converter in VHOST location on a server. To determine how national characters will be displayed on the server after conversion follow the link https://www.punycoder.com/. For example, create a domain in Plesk > Domains > Add New with a name тест.com. It's VHOST path on a server will be named
/var/www/vhosts/xn--e1aybc.com/ in Plesk for Linux and
C:\Inetpub\vhosts\xn--e1aybc.com\ in Plesk for Windows.