Whatever you want to use for the project. For a recent project, one of my coworkers created a domain specific language and the rest of us coded in that language

Whatever you want to use for the project. For a recent project, one of my coworkers created a domain specific language and the rest of us coded in that language. For another project, I use C++, because I'm leveraging an existing code base. Colleagues of mine use F# for program analysis tools, which they love. As you say, F# came out of the MSR Cambridge languages group, that's a great tech transfer story. That being said, there are a lot of internal libraries or projects written in C/C++ or C#. So I see a lot of those languages. I've also been learning ASP.NET, in part so I can better understand cross site scripting attacks on applications written on that platform.