My tuppence,

The language likely doesn’t really matter although Ruby/Python/Javascript seem to be the current language d’jour for this sort of thing.

Ruby/Python

Strengths

Interpreted languages; inherently cross-platform; support for GUIs (web-based and native); have JITs for performance; lots of community support and libraries/utilities to speed up development.

Weaknesses

Not installed by default on Windows; Installed by default on macOS and some Linux distros;

Javascript (HTML/CSS)

Strengths

Runs on a Node.js machine for native speeds; available in browsers; no additional software installation is required.

Weaknesses

Small variations in actual Javascript implementations will occasionally bite you; You will have to do some HTML/CSS for a GUI (command line is available via Node.js)

With regard to XML vs. JSON, it’s really up to what you want to do. JSON is a pretty loose standard but it is smaller and lighter than XML. XML (in various flavors) has more structural and type controls which may be important but those controls make the documents bulkier and require more resources to use.

Unless you are planning on building a huge business application, I would push you towards JSON and one of the scripting languages above, but it really is your call.

Regards,

Chuck