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?
The biggest tradeoff would be if someone was looking at using Unit Tests to figure out how a certain action may be performed, writing it in a different language would make it harder to use. Also you would have to make your C# code CLS compliant.
What are the (dis)advantages of writing unit tests in a different language to the code?