I agree with this to a point. You should produce code in whatever language you are most proficient/comfortable with, pending that language is suitable for the domain in which you are producing code in. I wouldn't want to try to produce a web application in Assembly.NET, nor would I want to develop a low level component in Ruby. Depending on the domain, language becomes a bigger issue, which may result in you not having the ability to be language agnostic. But, in the overall sense, I do agree with what you're saying.
