I have a pipe dream program to make that I want to be cross-platform. However, if I can develop it, it will save me thousands of hours of labor and is probably worth at least a million bucks. I need a language that will run into an exe and/or linux repo.
How computationally intensive is the program going to be? Do you have any idea about that?
Also if you don’t want people reading your source code, you probably want a compiled language, at least at the bytecode level like Java or C#, or perhaps a full compiled language. There are several very user friendly languages that are fully memory managed and produce fast binaries; D is my favorite, but Rust and Go are also very popular, especially Rust. D I particularly like, and a friend of mine is one of the leaders of the D community and wrote several libraries for it; its almost infinitely easier than writing C++ code, and also easier than writing C code, which I have to do * a lot * for my job, developing embedded systems, many of which are running on real time operating systems (in some cases I have to use assembler, or assembler in C, but in all cases careful manual memory management is required). When I don’t have to do manual memory management, I rejoice.
However, if you don’t need the performance, or the obfuscation of your code, that a compiled language provides (and of course, compiled applications like C++, Rust, D, Objective C, Java, C# and so on, can be decompiled), if I were you, I would use Python or one of the Javascript client side options like Node.JS since you already know that.
But if you’re good at Java, you should probably just do that frankly; its probably fast enough for your program, it runs on Linux and Windows and many other platforms, its GUI output has been fully modernized and is now superb, and also, you can use other languages on the Java VM, like on the .NET VM, such as Python, Scala and Clojure, which if you learn them can speed things up. Although I should say the Java implementation of Python is not guaranteed to be compatible with the vast array of libraries people have written for Python, which are the other major reason for using Python - it has a fabulous array of boutique libraries for doing absolutely everything.
However Python programs are scripts, so unless you take special measures, people would find it easier to steal your intellectual property, but they’re going to do that anyway if your software is as good an idea as you think, which it might well be.
Another thing you should consider by the way is that these days, it is increasingly important for Windows for software to be available via Microsoft’s Windows Store, their App Store rip off. It seems probable that Windows will eventually stop allowing users to easily install software not published in that way; I believe this is already the case on some platforms, just as Apple forces you to either jailbreak your iPad and iPhone or run only software from their App Store (and even the Google based Android phone is locked down, the open source Linux kernel notwithstanding).
By the way - you should also consider whether your application could be delivered via the web, because doing that solves the problem of portability to different operating systems altogether. Its also the only way to completely block people from being able to steal your backend code - if your web server is properly secured, since the backend code on a web application is hidden from users, so unless you encode your core application logic in the front end, which obviously one should not do in your case, it would work very well.