I think the right metric to measure here is not quantitative, but more qualitative.
In other words, how easy it is to adapt the language to the problem you are trying to solve.
So, in order to do that you need to have a good understanding of the principles each language is based on. Once you've got that, then you look at the resulting code and "measure" how easy it is to understand.
As was already said, this requires having someone with a very good understanding of the underpinnings of each language, which is not really going to be reasonable for most people.
The problem is that if you simply tried to port an idiomatic solution from e.g. C++ or Python into Haskell or Scala, or whatever, then you would probably end up with something very ugly, because you didn't adapt the language to your problem. You tried to force it to do something it wasn't necessarily designed to do.
In other words, how easy it is to adapt the language to the problem you are trying to solve.
So, in order to do that you need to have a good understanding of the principles each language is based on. Once you've got that, then you look at the resulting code and "measure" how easy it is to understand.
As was already said, this requires having someone with a very good understanding of the underpinnings of each language, which is not really going to be reasonable for most people.
The problem is that if you simply tried to port an idiomatic solution from e.g. C++ or Python into Haskell or Scala, or whatever, then you would probably end up with something very ugly, because you didn't adapt the language to your problem. You tried to force it to do something it wasn't necessarily designed to do.