This dialog can be opened using the Scripting language... command from the Scripting menu. It can be used to choose a scripting language for the current project.
There are two scripting languages available in QtiPlot: Python and muParser, which is in fact a parser for mathematical expressions. This dialog is available only if QtiPlot was compiled with built-in support for Python.
The default scripting language for each new project can be defined via the General -> Application tab of the Preferences dialog.