In terms of language choice, it is actually quite nice to see definitions of taxes, rules, and forms for different states and counties in a dynamic language like Ruby without verbosity of other languages like Java. As mentioned in another block post[1], when launching to new states, we paused and built tools to help us automate the verbose tasks, and Ruby really helps us there. We have good test coverage so we have high level of confidence when refactoring (which we do frequently).
In terms of JRuby, we are only using it on one of the components that needs to talk to a Oracle DB using JDBC.
We are early in the service oriented architecture path and will be learning (hard) lessons a long the way with mistakes and from new talented engineers joining us. What I am most impressed with this team of engineers is a collaborative culture and willingness to go extra miles for customers and for ourselves.
That said, I wouldn't be surprised that one day ZenPayroll technical stack would be polyglot.
[1] http://engineering.zenpayroll.com/benefits-of-writing-a-dsl/