No region found within the locale 'de' - exception due to empty locale with Zend_Locale_Format

[johnorourke]

The Zend class throws an exception if the locale is empty or not 5 characters long.

We are relying on CurrencyInterface->getLocale() to find the currency locale but it looks like this can be empty, or taken from the user's Accept-Language header. If the Accept-Language header contains only 'de' (not 'de_DE'), then the exception is thrown.

We should use the Magento locale resolver instead as it will always return a valid locale.

[johnorourke]

(I will send a PR for this)