A few times in my technical career/life I have tried to concentrate on one language, one stack (when doing web apps). This usually does not work out for me both because I have diverse interests and customers want me to use a variety of tech.
I have several deployed web apps using Clojure (and Clojurescript for one), one using Meteor.js, and a few Ruby/Sinatra apps for customers.
I am playing with Haskell/Yesod and I really like the type checking right down to web assets but due to my relative inexperience with this stack it does not yet seem agile to me. I have experimented with Pharo Smalltalk and Seaside quite a bit over the years, and to some extent Seaside and Meteor.js fill the same ecological niche for me (rich client web apps).
For general programming I also can't settle on one language.
I think that I am going to give up trying to settle on using just one or two languages and live with the overhead of staying spun up on different technologies.