[tswicegood]

I disagree that you have to pick one language and one language only to be really good at. At least after you've become a decent programmer. As a polyglot (I can start a project comfortably in PHP, Python, Ruby, JS, Erlang, and wouldn't feel too out of place working in Clojure or Objective C), I think a "master" programmer is someone who's gotten to that point to realize that a programming problem is a programming problem regardless of language. Language is only the syntax you use to formulate your answer.

[Someone]

I do agree with what you say, but there is a gamut with 'programming problems' on one end and 'work' on the other. Especially on the web, most of programming is work, not problem solving. When doing work in a language/framework, knowledge of other languages/framework can distract you. For example, a Java-only specialist would know whether substring takes start offset and length or start and end offsets arguments. I would have to look it up (or wait for the IDE to help me), but I do know that C# and Java disagree about it.