That rule is not a new rule. It just had fallen into a period where it was not enforced. After staff review it was determined that the switch back was too fast in some ways. As a result, staff will enforce the English only in the chats except not in townychat. That means it will not be enforced in mail, signs, or pm - onlt is the chats. At some date this may be tightened, but it is not expected anytime soon.
The Head Admins on the mini-games may choose to allow other languages as they see fit, but all staff are expected to enforce the English only in Towny chats (except townchat) rule.