I think what you're describing is "being an expert" rather than "being proficient."

If a project requires expertise in a specific language then sure, shoot for "an X programmer" (there are very, very few of these in reality). Otherwise "a programmer" (who is good) will almost certainly be able to perform the job, and do it well, all other things (e.g. motivation, interest in actually using the technology in the project, etc.) being equal.