From Xojo Documentation

Property (As String )
aWebSession.LanguageCode = newStringValue
StringValue = aWebSession.LanguageCode

New in 2012r2

Supported for all project types and targets.

When a browser connects to a web application and a new session is created, this property is set to the IETF Language Code provided by the browser. This code is used to help determine how dynamic constants will be rendered in your web application. One advantage to this system is that if you do not provide a translation for the user's language, you can offer them a choice and set this value to a supported language code. You can either use raw IETF language codes or the constants from the Localization module.