Jesper Louis Andersen:

I’m very fond of Erlang creator Mike Williams point: “If you don’t make experiments before starting a project, then your whole project will be an experiment”

If we instead ask about prototyping, then we need a programming language with certain traits. The team is usually small, so we need an expressive language, and we need to address the core kernel of the system in isolation, first. We don’t need a lot of interfacing to foreign systems and in general we won’t care too much if the system we build is fast. Also, we usually won’t need to operate the prototype in production, since it is simply a proof of concept.

There are more interesting points about language design in part one and two of this broad reaching interview.