Unless I'm missing something, I think most languages will be fine. I'd think pretty much every language will need to cross-compile their runtime library to WAS

Unless I'm missing something, I think most languages will be fine. I'd think pretty much every language will need to cross-compile their runtime library to WASM, which would include their garbage collection. C/C++ will obviously be easiest to map onto WASM, but I'm guessing other languages won't be too far behind. LLVM already has a WASM target.