You can just set the ServerName or ServerAlias-property to the punycode-value of the domain name, i.e. æøå.tld would become xn--5cab8c.tld. A converter is available at VeriSign.
After converting, save the apache2 virtualhost configuration file, (eventually run a2ensite config_file.tld , if you have chosen to config your web sites that way) and run:
The web server should start serving the web site immediately.