This bug can be reproduced manually with Zend Framework by the following code:
\Zend_Locale::findLocale('non existent')
The bug also can be reproduced in some live Zend Framework based websites, e.g.:
https://bionest-tech.com/payment/?lang=nonexistent
This website is not mine and is not Magento based, I have found it with Google by the «The locale '' is no known locale» query.
Magento 1 and Magento 2 both use Zend Framework 1, so they also vulnerable to this failure.
In my experience, the failure is often encountered in Norwegian websites: may be some Norwegian visitors have some specific locale settings (in a browser on in an operating system), and these settings are wrongly handled by Zend Framework (are absent in the Zend Framework locales dictionary).
The problem method is \Zend_Locale::findLocale
: