C++. If you are not familiar with it then you will be totally lost. This is also compiled, and great for native run times.
Chances are that if you’re trying to make something cross platform then pretty much anything consumer grade worth using has a web browser; if it doesn’t then you will be working in Assembly for embedded programming, or what ever standard libraries ship with the device.
It depends on what you’re making; if its not performance critical, then using a web language is a great choice for something very well polished with little dev time. If something needs the native run-times and high performance of a compiled language than it will be specific; QT framework would be a good choice for this; but you will quickly find yourself spending lots of time reading the documentation on how to do basic things if you’ve never done it before.
But if you’re doing it to learn something then sure go ahead if you want.