For many developers, the one-language notion is a sign of lack of professionalism. This is best exemplified by the Pragmatic Programmers' advice to learn a new language every year. The point here is that programming languages do affect the way you think about programming, and learning new languages can do a lot to help you think about solving problems in different ways.
From Martin Fowler’s at http://martinfowler.com/bliki/OneLanguage.html
I got the opportunity to participate in a java project since July. I’m going to share the pleasure and pain this brings to a .net developer. There are interesting differences in language, conventions, tools, application servers, IDE, … Since all I’m doing in the next weeks and maybe months will be java, you’ll hear that from me.
Too bad Martin fowler ended his paragraph with:
It's important to learn languages that are quite different in order to get the benefit of this. Java and C# are too similar to count.