add new string and mark existing string for deprecation

administrator/components/com_messages/models/message.php

@@ -364,7 +364,7 @@ public function save($data)
 			$subject  = html_entity_decode($table->subject, ENT_COMPAT, 'UTF-8');
 			$message  = strip_tags(html_entity_decode($table->message, ENT_COMPAT, 'UTF-8'));
 
-			$subj	  = sprintf($lang->_('COM_MESSAGES_NEW_MESSAGE_ARRIVED'), $fromName, $sitename);
+			$subj	  = sprintf($lang->_('COM_MESSAGES_NEW_MESSAGE'), $fromName, $sitename);
 			$msg 	  = $subject . "\n\n" . $message . "\n\n" . sprintf($lang->_('COM_MESSAGES_PLEASE_LOGIN'), $siteURL);
 
 			// Send the email

administrator/language/en-GB/en-GB.com_messages.ini

@@ -54,7 +54,9 @@ COM_MESSAGES_N_ITEMS_TRASHED="%d messages trashed."
 COM_MESSAGES_N_ITEMS_TRASHED_1="Message trashed."
 COM_MESSAGES_N_ITEMS_UNPUBLISHED="%d messages marked as unread."
 COM_MESSAGES_N_ITEMS_UNPUBLISHED_1="Message marked as unread."
-COM_MESSAGES_NEW_MESSAGE_ARRIVED="New Message from %1$s at %2$s"
+COM_MESSAGES_NEW_MESSAGE="New Message from %1$s at %2$s"
+; The following string is deprecated and will be removed in Joomla 4.0
+COM_MESSAGES_NEW_MESSAGE_ARRIVED="A new private message has arrived from %s"
 COM_MESSAGES_NO_ITEM_SELECTED="No messages selected."
 COM_MESSAGES_OPTION_READ="Read"
 COM_MESSAGES_OPTION_UNREAD="Unread"

language/en-GB/en-GB.com_messages.ini

@@ -4,5 +4,7 @@
 ; Note : All ini files need to be saved as UTF-8
 
 COM_MESSAGES_ERR_SEND_FAILED="The user has locked their mailbox. Message failed."
+COM_MESSAGES_NEW_MESSAGE="New Message from %1$s at %2$s"
+; The following string is deprecated and will be removed in Joomla 4.0
 COM_MESSAGES_NEW_MESSAGE_ARRIVED="A new private message has arrived from %s"
 COM_MESSAGES_PLEASE_LOGIN="Please log in to %s to read your message."