Swift currently supports Linux and has unofficial support for Windows. It's open-source and compiles via LLVM. Commercially Apple has been unenthusiastic about fleshing out its support for non-Apple platforms, but the open-source community has partly filled that gap and there's lots of enthusiasm in the community. Given the recent decline in Apple's hardware sales, they could be heading for a Microsoft-like rebirth into a company that's forced to play nicer with others.
https://forums.swift.org/t/swift-as-a-cross-platform-languag...