which programming language should I use for a multiplatform application

Question

I need to be really light and can make system calls

Answer 1

I'll say, use C# with Xamarin. C# is more powerful than Java.
Please note that Java & C# are multiplatform, provided you have JVM & .NET Framework (or Mono) installed. Otherwise, you have to compile to machine code, which will make your app single-platform.

Answer 2

JavaScript not only used in making front end webpages only.
Javascript can be used in back end web server with Nodejs.

JavaScript with electron, It can build multi platforms desktop applications run on Windows, Mac or Linux machines. Atom.io editor desktop application was built on JavaScript codes

JavaScript can also build iPhone or Android native mobile apps with React Native or Fuse Open.