When Microsoft released Visual Studio 2008, there was a thing they were talking a lot about at the conferences and in their online tutorials: the idea of writing the actual code in one language, and the unit tests in a different language. For instance, unit tests for C# code can be written in Visual Basic.
Six years later, I don't know any open source project which uses this practice and no proprietary project I've seen uses it. The subject is not discussed any longer at Microsoft's conferences, blogs or tutorials.
The benefit of this practice is that doing the same type of error in two different languages is more difficult. For instance, if I don't know that in C#,
3 and expect it to be
.NET Framework, specifically, makes it possible to use any .NET-enabled language to test another one. While testing C# code with Visual Basic tests doesn't look particularly interesting, given the similarity of both languages, I would imagine that, for example, testing F# code with unit tests written in C# could be useful for people who are not that familiar with functional programming.
Personally, I have no experience writing unit tests in a different language. Therefore, I don't know if there are actual drawbacks in this technique. I often write system and acceptance tests in a different language (for instance in Python for a Node.js web app, because Python's Requests library is so great), but this doesn't count, since those tests are truly language-agnostic: they just do black box testing through HTTP: I can rewrite an app from Node.js to ASP.NET MVC, and the tests won't even notice the change.
So what could be the drawbacks, and why is this practice not that popular?