# aptitude install roundcube
# cat /etc/apache2/conf.d/roundcube > /etc/apache2/sites-available/webmail.conf
# vi /etc/apache2/sites-available/webmail.conf

<VirtualHost aaa.bbb.ccc.ddd:80>
ServerName webmail.DOMAIN.DOM
Alias /program/js/tiny_mce/ /usr/share/tinymce/www/
Alias / /var/lib/roundcube/
...
</VirtualHost>

# vi /etc/roundcube/main.inc.php

$rcmail_config['default_host'] = 'localhost';
$rcmail_config['language'] = 'de_DE';
$rcmail_config['date_format'] = 'd-m-Y';
$rcmail_config['create_default_folders'] = true;

# a2ensite webmail.conf
# service apache2 reload