There is pretty bleeding-edge support for compiling to both asm.js and WebAssembly in Rust that recently landed: https://users.rust-lang.org/t/compiling-to-the-web-with-rust...

So, at the very least there's support for C, C++, and Rust at the moment, though of course since the standards aren't yet finalized and the toolchains are evolving how well it works at any given moment may vary. I'm sure there's work going on for other languages to compile down to WASM, just not sure of the status of any other languages at the moment.