If a Java, C#, C++ (or F# or OCaml) programmer isn't able to pick up Scala (or any other typed object oriented language) quickly on the job, they aren't that great of a programmer (even if they are experienced).

Same for knowing any of {Perl, Python, Ruby} and being able to easily pick up the other. Some of the more advanced idioms may take longer to learn, but it's a fact of life that you'll have to learn new languages as you go.

On the other hand, if you need to hire average but experienced programmers (e.g., if you're doing outsourced CRUD app development) then you might have an argument.