Add a constant that contains the current interface language

The cross-platform ability to determine the current installation language from the script can be very useful for multilingual applications. For example, you can set language-dependent components (translations, manuals, etc.) for installation by default during the installation phase. 

Below is an example of how to use it in a setup script.

// code placeholder
Component.prototype.isDefault = function() {
  return installer.value("UILanguage").substring(0, 2) === "de";
}

Attached patch with the implementation of the functionality in the source code. ui_language.patch