C# must be top contender here, with Visual Studio. I reckon this will give the best GUI, and a good way to link to the SKU scanner device drive. Plus the audio out beeper and cash drawer I/O
But I’m sure Java and Java FX would do a good job.
C++ would do it. Something like Qt (or a contacts’ NeoGFX) GUI library
Other ideas are do it as a self hosted web app running locally on the post till, then use the browser. Many languages here.
The limiting factor is how to access the custom post APIs for the till hardware. Spike that out before you firm up your decision.