The main benefit of knowing multiple languages isn't in writing them directly. All other things being equal, I'd rather work with a C# programmer who also knows C, Python and Lisp (for example) than one who's only ever hacked in C#. It's not that knowing more languages is better, it's that being able to think about problems at multiple levels and from multiple perspectives is really helpful.
A programming language that doesn't change the way you think about programming is not worth knowing. -Alan Perlis
It's not about checking off one more language, or putting it on your resume; you just need to understand its underlying concepts well enough to program in it to get the full benefit. You won't get that from having a basic understanding of the syntax.
The more direct answer is "it depends".
At larger companies you're expected/allowed to specialize, but as I said above, I believe there's still benefit to understanding things beyond your one favorite tool.
At smaller places, you really can't get away with that. If nothing else, you typically need to maintain your app as well as build it, and you probably souldn't use the same languages for running through logs/data munging as you do to actually build your app. I guess you could technically get away with knowing a single language, but the benefit of having a well-performing, strongly-typed (or at least assertion-capable), probably compiled language do the heavy lifting, and a scripting language for maintenance/setup/scripting tasks seems pretty big. I wouldn't want to do without it, certainly.