making lang::message::register refuse to register a message for a key that doesn't exist unless it's for the en_US locale. This is a precaution for the case where non en_US locales have somehow corrupted keys or keys that have been forgotten to be added to en_US
adding the columns creation_user and creation_date to lang_messages. At least on the translation server those are useful. Those columns replace the table lang_messages_created in the messages-audit.sql file (now removed)