Wordpress.com is the hosted version of the open source wordpress. You can’t install new themes or plugins on WordPress.com. But if you would like to buy a theme and install it on your own host you are on the right place, this gives you uniqueness and flexibility.
Once you have installed wordpress on your website, there are two ways you can install the new theme – from the wordpress theme manager or manually via ftp. For detailed steps plese refer to the theme documentation included in the archive file which your received when you bought the theme.
All of the wordpress themes are localized using GNU’s gettext functionality. This means that each of our themes includes one .pot file from which you can generate .po file which you can edit to translate the theme to the language, which you want.