Frankly, I’m a bit tired of hearing the hammer and nails metaphor and “right tool for the job” from developers when they talk about programming languages. While there’s nothing wrong with selecting the right tool when there clearly is one, I think the phrase is often misused and seems to be a cheap explanation to justify personal preferences and subjective decisions seemingly objectively. I think when that phrase is uttered, it mostly means something like: the right tool for me (or my company) at that particular time, considering my (our) experience, knowledge, skills and preferences at that time.
Most of the times there are always several “right tools” that are probably equally good “for the job” and the reasons for choosing one over another are usually not merely technical in nature but depend on many other variables from the larger context in which the choice is made. Actually, I don’t even think there’s ever a completely right choice. The complexity of the real world makes it impossible to know for sure, but nevertheless, choices must be made and we approximate. If someone else made a different choice in a similar situation that doesn’t mean it was the wrong tool for the job.
But usually you only need one general purpose language. There are of course some differences between them and some languages have grown into certain roles (forgive me some sweeping generalizations): C++ is for performance and hardware access; Java is for applications and information systems; PHP, Python, Ruby for web apps and scripting etc. (as a side note, I think Scala can play both of the latter roles) In some cases you might perhaps want two of them — programs in higher level languages sometimes “drop down” to C++ for performance, Grails mixes Groovy and Java — or maybe even three, but much more than that would be overkill. Note that I am not at all against the polyglot programming meme, but I do think the choice languages for any given project or organization should be limited to a few.
So programming languages are like tools in the sense that choosing a language somewhat limits what you can do with it. But actually, I would say that a general purpose programming language together with it’s libraries is more like a complete tool set than a single tool. You can choose any tool set you like, each will get the job done. One set might be missing a hammer, another might be missing a saw — if that turns out to be a problem for a few situations then you compensate. Perhaps by renting a specific tool for just the time when you need it, deciding on using two tool sets from the start, dropping a requirement, or finding a workaround that is doable with your current tools. Even if you have to do the latter, it still doesn’t mean you are looking for nails where there aren’t any — it might be a valid choice considering your constraints. However, if this keeps happening then you probably have chosen the wrong tool (or someone chose it before you).
But besides being a tool set, a programming language also forms an important base for communication between developers, just like natural language does. With this comes a whole new set of considerations for choosing a language that have nothing to do with how exactly it fits to solving a problem technically. It also determines what kind of community you have access to, what kind of cultural values you may be indirectly associated with, how many “compatible” developers there are in your area etc. And I think sometimes, but not always, these questions may be more important than the technical ones.