Create a .tpl file in template/ and a lang.php file with the same name in language/.
Edit function porg_get_pages() in include/functions_piwigodotorg.php and add your page to the list
Changing language using the footer is url based, to test another locale change $user['language'] in main.inc.php
// adapt language depending on url
function porg_user_init()
{
global $user, $page;
$page['porg_domain_prefix'] = '';
- $user['language'] = 'en_UK';
+ $user['language'] = 'fr_FR';
[...]
}