The best language is: one you know.
If you don't know the language and the framework, then you add a large learning curve to your task.
Quote:
My project is a rental desktop app ... I was thinking about NW.js and Electron.
But ... anything javascript is probably a bad choice for a desktop app: this is where desktop languages really do well. So if you know C#, C++, or even VB I'd recommend you go with that.