Drupal 7 Commerce: How to change cart page title?
There are probably a dozen different places you could do this, personally I'd go for hook_page_alter() in a custom module:

function MYMODULE_page_alter(&amp;$page) {
  // Make sure you're on the right page
  if ($_GET['q'] == 'cart') {
    drupal_set_title('New title');
  }
}


You would probably get away with doing the same thing in your theme's template.php file (maybe in hook_preprocess_page()) if you're not keen on writing a custom module just for this functionality.

The alternative to this is to install the String Overrides module. I've used this module in the past to override the title for exactly the same page (/cart) and it works great. It also means you can take your translation strings out of the settings.php file, which in my opinion isn't necessarily the best place to have them (even though it's absolutely a 'Drupal' way to solve the problem).