The only thing fair is to give one team 6 months (ideally with a couple contractors who are experts to teach them best practices) to try the new thing. Less than 6 months and they are not far enough along the learning curve to make any useful statements. If you want a work done comparison you need to give half your teams (minimum 20 teams, ask a statistician for more details if you want to go lower or could go higher) that 6 months, and then after that give them a year to use the new one in real work, and compare that final year.

You're perfectly right that it would be awful to be pitched against each other in this way.

So I recant my suggestion, but maintain my critique about using lines of code in a single project as somehow validating a language choice. There are too many variables. And, more to the point, the refactored code will almost always be much better, regardless of the language selected.

But I hope most people here don't feel a morale hit when their code gets tossed. Inherently, I understand the desire for one's creative work to live on, but that's essentially incompatible with this industry. And I also have found much joy in turning-down/deleting existing services I've written. My team celebrates these events and it's always fun to give that high-maintenance service a viking funeral.