By disabling the festival option if the festival program is not installed, the dialogue defaults to External Program, the most commonly used option on Windows and MacOS.
Windows and MacOS commonly can use multilingual system voices, therefore you do not need Festival unless you require a voice for a region that are not available in the system voices catalogue
The scripts for setting up the dialogue use with notation to make the code more readable.
The main dialog uses a compatible group tag for the option radio button and uses a tab order that groups the radio buttons together.