As an english-speaker, I don’t understand the concept! I18n will definitely barf on arbirtrary locale codes. It already barfs on fr_FR, which is a valid code (it only accepts fr). A new layer would need to be added to do some mapping.
I don’t know what the solution is in the meantime… A decision about formal vs informal within the language team at the Transifex project should be made so that translators know what they’re supposed to use.