WHMCS billing system allows selecting another language (dropdown menu on the bottom of the page).
But how to make homepage or other custom pages to show in another language too?

there is some guide: http://www.blog.modulesgarden.com/automatic-currency-and-language-setup-for-whmcs/
On the bottom of the article, there can be downloaded this Language Hook. the hook .php file should be added into /includes/hooks/directory

There are some mods for language (search "language" in mods database)