Actually, I would disagree with you, based on (1) my personal work and (2) discussions with a variety of other people as I thump the "not all programming needs to be imperative languages" drum.
First, working in a single language allows you to accumulate what, for lack of a better word, I call "IP". Components/libraries/frameworks; a body of work. Having a single language gives you the leverage of previously written code solving prior problems in a debugged fashion.
Second, having a single language allows easier social operation; people can review each others work, a common body of knowledge can form around the language under common use which is difficult to maintain for multiple languages simultaneously.
Third, having a language which is a bit of a melting pot allows idioms to be used in which people are comfortable with their specific idiom - OO/FP, etc.
