Choosing the appropriate language for the situation

That is the real difficult question, IMO.

In my experience, the language gets usually dictated to me by some factors. I get more re-design website jobs than anything for some reason, and for that reason, the client already has a hosting server and is not willing to change, or will only accept to switch into a new server after a certain date. That comes attached with what languages are available, for most of my clients are smaller companies who have multi-tenant host available to them, like Bluehost, GoDaddy, Hostgator kind of services. Some of those companies haven't heard of cloud computing yet.

Sometimes if you suggest moving on from PHP into Ruby it may mean they have to switch hosts or pay more for their hosting, which is why some clients will be resistant to different languages.

If you're the one picking, and you are in liberty to choose the host and everything, you can pick one you like, because it is most familiar to you or make your choice based on features available, and whatnot. If you offer to move the files and handle the whole migration for the client, they are often a lot less resistant to change... And if you can offer free hosting for a year somehow, they would have a real advantage in the switch.

Rarely there will be a case (in smaller companies at least) where the language actually matters. Maybe you need to interact with a specific kind of database that is shared by other applications, like a stock app, or whatever. In those cases, usually there is a local programmer who can help you understand their needs and might be willing to give you a hand or a starting point in the migration and will often be able to discuss what's the best language with you.

Camilla said a lot of truth of the situation there. I would also point out that using Ruby on Rails as a framework doesn't keep you from using JavaScript in your application, nor PHP. A combination of the three, or different combinations of any languages can be beneficial in many cases. Working with many different languages and integrations of languages and feeling out how you fit them together is beneficial for you as a developer to begin to piece together what languages you feel work well together.