I don't think this is possible out of the box. You could modify/extend the block to implement database resources.
Another approach that I've used is to have a token (e.g. CAR_NAME_MANDATORY) as the MessageTemplate property and then before displaying the message template loop through all ValidationResults and perform a lookup from an external source to retrieve a localized
string version. Not sure if that fits with your design or not, though.
Enterprise Library support engineer