Zend_Currency is also able to work on a given country by using
$currency = new Zend_Currency('US');
// See the actual settings which are fixed to 'en_US'
When you know that you are using a territory, then you should uppercase it. Otherwise you could get an in your eyes false locale in return. For example, when you give "om" then you could expect "ar_OM" to be returned. But in fact it returns "om", as it's also a language.
Therefor always uppercase the input when you know that a territory is meant.