Apple.com style Localization
I had some helpful advice via Craft's support channels. Here was the solution I arrived at with their assistance:

You can hard code the locale in index.php in the doc root.

define('CRAFT_LOCALE', $locale);


The normal approach seems to be that multiple index.php pages can be used in a multi-locale site. I prefered to keep the one, so I used this tiny hack that derives from the url what the desired locale is:

&lt;?php

// Path to your craft/ folder
$craftPath = '../craft';

$url_folder = substr(substr($_SERVER["REQUEST_URI"],1), 0,
    strpos(substr($_SERVER["REQUEST_URI"],1), "/"));
$locale = 'en_nz';

if($url_folder === 'worldwide') {
    $locale = 'en_zz';
}
else {
    $locale = 'en_' . $url_folder;
}

define('CRAFT_LOCALE', $locale);


This hack is confined to the index.php so it seems pretty clean.