MySQL is still a key part of our systems; although MongoDB covers many unrelated persistence mechanisms due to its ease of setup, the transactional power of MySQL is key for a payment system. Race conditions security issues that would make MongoDB unsuitable are handle easily by MySQL transactions. So add SQL in the MySQL dialect to the list of languages to speak.
Last but not least, let me say we aren’t installing Rails anytime soon. However, on the DevOps side our system administrators are starting to work with Ruby as an higher level of abstration with respect to shell scripts. For example Chef, a tool for server configuration management, uses Ruby as its language for specifying which libraries and daemons a new machine should contain.