Use the DefaultLanguage property to specify the default message set for the MessageManager. When you use AddMessage to add messages to the MessageManager, and do not specify a message set for those messages, they are associated with the message set specified in this property.
The DefaultLanguage property is initialized in the Global.asa file as follows, where "usa" references a message set that has been added to the MessageManager using the AddLanguage method:
MessageManager.DefaultLanguage = "usa"
Do not use this property to switch to different message sets inside an application. Instead, specify the MessageSet parameter for each desired call.