The solitary programmer that loves his work (a hacker) can choose the best language for the task. Most working programmers have very little control of the language they will use. Generally, this issue is dictated by pointy-haired bosses who are making a political decision, rather than a technological decision, and lack the courage to promote an unconventional tool even when they know, often with first-hand knowledge, that the less accepted tool is best. In other cases the very real benefit of unity among the team, and to some extent with a larger community, precludes choice on the part of the individual. Often managers are driven by the need to be able to hire programmers with experience in a given language. No doubt they are serving what they perceive to be the best interests of the project or company, and must be respected for that. However, I personally believe this is the most wasteful and erroneous common practice you are likely to encounter.
But of course, things are never one-dimensional. Even if a core language is mandated and beyond your control, it is often the case that tools and other programs can and should be written in a different language. If a language is to be embedded (and you should always consider it!) the choice of language will depend a lot on the culture of the users. One should take advantage of this to serve your company or project by using the best language for the job, and in so doing make work more interesting.
Programming languages should really be called notations in that learning one is not at all as difficult as learning a natural language. To beginners and to some outsiders 'learning a new language' seems a daunting task; but after you have three under your belt it's really just a question of becoming familiar with the available libraries. One tends to think of a large system that has components in three or four languages as a messy hodgepodge; but I argue that such a system is in many cases stronger than a one-language system in several ways:
There is necessarily loose coupling between the components that are written in different notations (though maybe not clean interfaces),
You can evolve to a new language/platform easily by rewriting each component individually,
One language may not be a good fit for the overall system - having multiple languages for your modules allows you to pick the right tool for the job.
Some of these effects may only be psychological; but psychology matters. In the end, the costs of language tyranny outweigh any advantage that it provides.