For data science, python is the strongest language right now due to massive library support for anything you want to do. Its also the easiest to understand and find developers for (from your list of languages).
Go is also nice and will bring you support for types and high er runtime performance (but slower development speed and maintenance speed).
Haskell will be hard to find developers for, but the ones you find will be very good. But getting a team of 20 to use haskell will be a great challenge, and also the library support for data science is tiny.
It sounds like they already have a data science team, and they'll continue using Python. The OP seems to be talking about building a team of non-data-scientists to do UI, data fetching, and other user facing stuff. I don't think suitability for data science is a factor.
Go is also nice and will bring you support for types and high er runtime performance (but slower development speed and maintenance speed).
Haskell will be hard to find developers for, but the ones you find will be very good. But getting a team of 20 to use haskell will be a great challenge, and also the library support for data science is tiny.