I think WebAssembly will (eventually) be that language. Write in any language, but compile to one target that runs on any device.
The bigger challenge is the integration with the host OS - we need a common set of OS calls that can be made available to applications running in WebAssembly (eg, WASI).
WASI is the tip of the iceberg for a UI application; its scope doesn’t extend outside of the kind of things you can access with your friendly neighborhood libc.
The bigger challenge is the integration with the host OS - we need a common set of OS calls that can be made available to applications running in WebAssembly (eg, WASI).