The traditional method of recruiting software developers is to go "We are writing our program in C++ so we should get a guy who does C++". I imagine many recruiters and companies use keyword filters on CVs to filter out candidates who don't have experience in the particular language used on the job. The basic premise of this post though, is that this is not necessarily going to get you the best man for the job.
My main points are:
- The time needed to learn a new language is shorter than it was and isn't that long
- 3 years doing 3 different languages is a better sign of a good and keen programmer than 3 years of 1 language
- The overhead of learning a new language becomes a smaller proportion over time and is overridden by the extra productivity of a "better" employee
- If you've only hired say, Ruby developers and then stop doing Ruby you can be in trouble
A collegaue of mine recently said that you can grasp the basics of a language in about a month and start being confident and expressive in about 3 months and I agree. After that roughly 3 month stage, you start seeing diminshing returns on learning more particulars of that language (perhaps an example of the Pareto principle). The time taken to learn a language has dropped over the years, thanks to the wealth of information available on the internet in tutorials, blog posts and places like StackOverflow, as well as the reduction in the costs of publishing meaning more people are able to produce books on specific programming languages.
Now obviously, someone who's experienced in a language is going to be quicker to start writing in it than someone who isn't but that initial overhead will become a small proportion over the course of a long term employment (and hopefully you're hiring the kind of people you want to keep for a long time). The 3 month timescale I suggested above is obviously up for debate, but the argument here is based on the assumption that multi-language programmer will turn out to be more productive than the single-language programmer and thus cancel out the overhead, however large it is.
My final point is that hiring language specific programmers can seriously backfire if you decide to stop using the language. Kevin Rose of Digg raised this point at LeWeb conference where he said about hiring PHP developers to build out the Digg site "you end up with lots of PHP developers, but at some point, PHP isn’t a problem anymore and you are stuck with all of those developers". This is especially relevant I would think for startups, whose business direction is not as fixed as large corporations and may decide to pivot from say web development to iOS development and therefore suddenly need to be able to program in a different language.