Cloudflare announced this in a blog post a few months ago (https://blog.cloudflare.com/cloudflare-workers-now-support-c...). They explain a bit more about the history of COBOL and why this specific project exists.
We've announced a few more languages for Workers: https://blog.cloudflare.com/cloudflare-workers-announces-bro...
Pure wasm doesn't work yet since the VM needs either C++ exceptions or longjmp (to unwind). Wasm exceptions support will eventually help there, but meanwhile there is a dependency on JS exceptions.