I do believe that this type of message is the worse an most annoying kind of message they could sent. I rather have an email sent to my account with this spam then that they put it full screen in the game (although an email would still be too agressive for my taste).
Also..why is it needed to broadcast it to my screen in different languages (atleast on EU-servers)?? just follow the client language setting.
My advice is to hard code an expiry time into notifications and sent all players a notification (in their client language) with the spam. Also, when the offer is over, the notification self deletes.
That way we can decide for ourself when we read it and more important, also to ignore it cause we allready know bout the offer and are not interested.