I'm using different languages for different parts of the system.
While I could probably do everything in one language, I'm not sold on the idea that that's actually a good idea (aside from developer convenience, which considering I'm the only developer I don't rate very highly). I also don't feel qualified enough as a language person to really hold defensible opinions about their relative strengths and weaknesses yet.
For example, part of the system is a network service that runs deployed on servers in our environment. All that code is Haskell. I didn't want to have to deploy and manage an interpreter or JVM on those systems, and I don't trust myself to write C that I feel confident deploying in production. Haskell seemed like a good win there (other folks might use Go, or whatever). It turned out to be really easy to write a network server in Haskell that receives the messages that the service needs to receive. Also, it was the first time I used a language that had good (or really, any) pattern matching, and I'm sold on it.
The "business" logic of the system is all currently in Racket. It could certainly have been written in Haskell, but I'm still getting up to speed in it, whereas I was able to get pretty minimal functionality up and running with Racket very easily.
Right now only the web interface is Clojure. I briefly considered using Yesod for the web piece, but it seemed really complicated. And although Racket has a nice web server, it requires buying into the "continuations" style that they advocate (or rolling your own web libraries, which seemed like a bad idea for this project).
I think the continuation-based approach for web servers is certainly valid, and maybe even technically better, but they're swimming against the tide with that approach, and as web programming is so far my least favorite part of building this system, I wasn't willing to buy into that enterprise.
Thankfully, I was able to get a Clojure web app up and running very quickly (basically just Ring and Compojure).
The system is designed where there's pretty hard boundaries between all the parts, so by the time all is said and done, it might all be written in the same language; but that's not necessarily a design-goal. I figure different languages seem to have different strengths, and there's not really a good reason for me to try and shoehorn everything into one.