Complete the list of english locales

English locale is used in more countries than the US.
https://pic.dhe.ibm.com/infocenter/ratdevz/v7r5/index.jsp?topic=%2Fcom.ibm.etools.est.doc%2Ftopics%2Frxmlenaloccp.html

Since this is a library used in multiple projects, I believe it should contain at least the most common locales.
(I encountered a problem using "en_GB" locale before proposing this change)