Now that there are several commands, dotnet repl and dotnet new, that supports language arguments, I think it makes sense to be able to somehow specify a default language.

If you primarily work with F# projects, I think it'd be nice to not have to specify --lang fsharp every time you want to create a new project or start the repl.

Is there an existing configuration mechanism in place?

I propose a simple DOTNET_DEFAULT_LANGUAGE environment variable that you can set to [c#|cs|csharp|f#|fs|fsharp].