This module demonstrates how easy it can be to adapt existing code in OSC.
All I had to do was add one single line to define_languages.php (admin) to make sure it always looks for the shippingpage.php file. Obviously I had to then implement it in the Admin & Catalog side, but that's not a big deal as you've just found out.

You can edit the contents of this file from within the Admin ( -> CATALOG -> DEFINE SHIPPING)

If you can't open/edit the /[language]/mainpage.php file you should probably set the right permissions. You'll get a warning anyway.hhhhh