Where are page specific language file included?

Can someone point me at the piece of code that includes page specific language files please?
I'm trying to make a jQuery mobile version of my store. On the whole, it is pretty straight forward....most pages just work.

One problem I've just noticed is it seems that the loading of page specific language files isn't working correctly.
It seems to be including all language files that begin with the current page name.
For example, if I load the checkout_shipping page, I seem to be getting the checkout_shipping AND the checkout_shipping_address language files loaded.

I'm pretty sure this will be down to something I've done!
If you can let me know where the language files are pulled in, I should be able to see what's going on.

Thanks!

it's ok I've found it...includes/modules/require_languages.php

It seems this does include both files as I thought.
If I leave the echo in this section of code

// set language or template language file
$directory_array = $template->get_template_part($language_page_directory . $template_dir_select, '/^'.$current_page_base . '/');
while(list ($key, $value) = each($directory_array)) {
  echo "I AM LOADING: " . $language_page_directory . $template_dir_select . $value . '<br />';
  require_once($language_page_directory . $template_dir_select . $value);
}

I get...
I AM LOADING: includes/languages/english/template/checkout_shipping.php<br />
I AM LOADING: includes/languages/english/template/checkout_shipping_address.php<br />

...is there a reason for loading both language files?
In other words, is it normal Zen Cart behavior or something I've broken?

It's normal.

Thanks Doctor B,
Do you know if it will cause any problems if I change it to load only the 1 specific page language file?

Just out of curiosity, why does it load both language files?

What problem is it causing for you right now?