This depends on how your server is setup. If you're using a
NameVirtualHost for your site, it's easy - you just add the following line:
ServerAlias example.ru example.de
to the configuration for your virtual host.
If you're not using virtual hosts, you don't need to do anything - apache will serve the same content regardless of what domain name is used to connect to it.
Of course, you also need to make sure that the DNS records for example.ru and example.de point to the same server as example.com.
If you are using SSL for your website, you need to have a certificate with Subject Alternative Names for all domains you are serving.