This is true for what pg calls Blub programmers who get stuck in idiomatic ruts, but there's so much idea-sharing among the current popular high level languages that I think once you master a certain number you can pick up new ones fairly easily. The current ecosystem is one in which the really important idioms--fundamentals of good software--translate well from language to language.
Exceptions might exist for things like APL, and to a lesser extent "extremist" languages like Haskell, Lisp, Forth, which require paradigm shifts to think about effectively.
Almost everyone starts as a Blub programmer -- "Blub" was BASIC when I was a kid and that's what I learned first -- but that's beside the point.
You're right in that the more languages you learn, the easier it gets to pick up new ones, but that only happens if you make an effort to master them, so you can learn the thought processes behind them.
Of course, after a while you'll hit the point of diminishing returns, where most new languages you learn won't have a boatload of new, eye-opening, paradigm-shifting concepts for you to discover and learn. However, there are two things to keep in mind:
1) You never know when a new language will teach you something important unless you try your best to learn it properly.
2) If you think you're going to spend a lot of time working with the language X or if you're going to dive into a huge codebase written in that language, then by all means try to master it. The better you are in that language, the less code you'll write in it and the less code there is to maintain, the better.
Exceptions might exist for things like APL, and to a lesser extent "extremist" languages like Haskell, Lisp, Forth, which require paradigm shifts to think about effectively.