RequestContext::getMain()->getLanguage() is a hack that we shouldn't rely on, as ZObjects should be independent of the current request context; instead, let's pass the requested language as a parameter.

• ZError::getErrorData() should pass a Language input to ZObject::getHumanReadable()
• ZObject::getHumanReadable() should throw if a Language is not passed in.
• ZKeyReference::getKeyLabel() should take a Language input, like ::getHumanReadable()