Earlier quoted context omitted.
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 langua…
I dont want to pay people to 'pick things up on the job'. I want people with a proven track record in the language we use.
Google would disagree. As would Microsoft (they were very interested in interviewing me, even though I've never done Windows or C# programming and made that clear on my resume), Facebook, Amazon (http://groups.google.com/group/mi.jobs/msg/d81b6c1fa8f361fc) and other serious software successes.
Sounds like you want coders, not developers. What reason would talented developers (the ones with offers from all of the above, plus start-ups competing with you) to join you?