Unit tests have different requirements than production code. For example, unit tests may not have to be as performant as the production code.
Perhaps it sometimes makes sense to write your unit tests in a language that is better suited to writing unit tests? The specific example I have in mind is writing an application in C# but using IronRuby or IronPython to write the tests.
As I see it, using IronPython and IronRuby have several advantages over C# code as a testing language:

Mocking can be simpler in dynamically typed languages
IronPython has less verbose type annotations that are not needed in unit tests
Experimental invocation of tests without recompilation by typing commands at the interpreter

What are the tradeoffs in using two different languages for tests and production code?
Main disadvantage as I see it is maintainablity.  If you code in C#, your development team are competent in that language, as will new hires be.  You need a multi-functional dev team.
I think that it's also worth noting that you probably don't want people writing their tests in a language that is maybe not their strongest.  Your test code needs to be robust.
You also need to be switching between syntaxes whilst writing codes/tests - this is a bit of a nuisance.
What are the (dis)advantages of writing unit tests in a different language to the code?