This gets pulled from the MessageLocaleID set in the application start up. So generally, when using this, it applies to the application pool instead of the session. However, there are some things that you can do. The first, you will need to control the locale ID by using the "Retrieve..." methods to pull the localization messages:
MicroFour.StrataFrame.UI.Localization.RetrieveTextValue("yourKey")
You can then store the locale ID in a session variable instead of using the core application logic to pull this. One thing that I have done before is subclass a label, etc. and have it look at the session variable that stores the locale ID and the pulls it and sets the label. This allows you to use the localization engine, but takes the setting of the label out of the frameworks hands and lets you manipulate this to your needs. That is why these methods are exposed.
Hope this helps. 