I'm helping a software team to embrace a transition to DevOps and I realized that they struggled a lot with testing. The one point that troubles me the most is that they absolutely want to write unit tests in pytest, when the application is developed ... in C#. Part of this excentric idea comes from the fact that they are convinced that it's the QA's job to write unit tests (who don't know anything about C#), but while I do see how to effectively explain that it's the developer's job to write unit tests, I'm struggling to explain to them why they shouldn't try to write unit tests in a different language than the one used to develop the product.
It just seems so obvious to me why doing it any other way would be detrimental to the project that I don't even know where to start. I tried to Google it to find inspiration but I couldn't find anything on that topic. Maybe I'm just plainly wrong, but never have I seen this before, even when I was a developer. And now seems like a good time to address what I think to be a misconception, given that they are only getting started with unit tests.
Which arguments could I use to explain that writing unit tests in a language that's different from the codebase to be tested is a bad idea?