Why does it matter whether the solvers themselves have a common language if it's possible to make interfaces to solvers that expose a common language?
It's the common language that matter more though. I've used GAMS in my undergrad, so I'm under the impression that it's had at least some adoption, but it's proprietary AFAIK. I've know some people who use AMPL too, which also seems popular, but I'm not sure about it's licenses.