How to choose a language for a program like Visio, Xmind, etc.?

Question

Graphic arts

Slightly familiar with C++, C# and Java.
I want to try to write a program for creating flowcharts, mind-maps and similar graphics. I will write to build up experience and coursework in computer graphics, but if it comes out worthwhile, then selling is not a sin.
It's too early for me to think about cross-platform, so for now only Win10 is planned.
Is it necessary to know graph theory from discrete mathematics?
Can you write for Win10 in any language or is there an optimal one for such a task?
Is it possible to write for a cross-platform in any language or is there an optimal one for such a task?
any of these languages: C++, C#, Java, Phyton, HTML5
cross-platform platforms: Win7-10, Linux, MacOS X, Android 6+, iOS 11+, web version