If someone feel completely lost when he learns a new language, even if he specialize, that would scare me.

If a company want someone to do a quick job in short term (1 week – 1 month let’s say), then you might want someone who can be productive technically from day 1. Still you don’t eliminate the non-coding task like analysis, learning the business domain, etc. and even further, you often have a learning curve of how they used the language(s) in their architecture (coding style, framework, philosophie, etc.). In other words, only knowing the technologies underneath is far from guarantying someone to be productive from day 1 when diving in codes.

It might be reassuring for a lot of people if the ressource know the techno, but if you are looking for a long-term team member to bring the team to the Stanley Cup (I’m Canadian, eh!), I would suggest to take the best developers instead of only the ones who know the technologies upfront. Easier said than done to evaluate, but you can evaluate what you search for without necessary focus on the technology you *currently* use. I may be and/or sound judgmental, and I do know some people on the market who still struggle with this, but for example, if your future developer can’t solve a simple recursion problem, in the syntax he wants, then it smells bad. There is other factors than skills or potential skills that might make someone fail those kind of tests of course.

All in all, I strongly believe that a good developer, with reasonable time for it, will learn the technologies and in the long run should make your team way more productive than an average one. So my suggestion would be that if you got a very interesting resource that don’t know your techno, find a way to evaluate his high-level/abstract/problem-solving/etc. skills, and take a decision based on this rather than the techno he had the chance to touch in his past.

Lot of people also suggest to learn/experiment-with a new language regularly (e.g., one per year), as you can see different ways of doing and become a better programmer in your language of choice. Similarly to playing a lot of sports will often make you better in your favorite one as you will develop other skills that you may use creatively in your chosen one(s).