Charles,
If you’re using /wp-login.php?action=register as the registration page, then that’s the default WordPress registration page. s2Member hooks into it in a couple of different ways to add Custom Registration Fields that you’ve created, but the rest is controlled by the WordPress core software.
If you’re using the s2Member Pro-Forms, then you can modify the HTML templates in wp-content/plugins/s2member-pro/includes/templates/forms/ to customize the output of how things are displayed when showing a Pro-Form. Anything that isn’t customizable through the templates will need to be customized by hacking into the code or writing custom code that uses the WordPress hooks feature (there are lots of s2Member hooks available) to make customizations.
Using the Pro-Forms themselves is simply a matter of generating the necessary form and then copy/pasting the resulting shortcode into a WordPress page (a page you might want to call “Register”). Please see the Pro-Forms Quick Start guide in the plugin for more info: Dashboard -› s2Member® -› PayPal® Pro Forms -› Quick Start Guide.
Finally, as far as translating things and changing messages, you might also want to look into changing front-end messages and experiment with those.