Even though it is technically possible to develop plugins for ReadyAPI in any language that targets the Java Virtual Machine or the scripting API (for example JRuby, Jython, JavaScript, Clojure), we advise against it and recommend using one of the currently supported options, Java and Groovy.
Java
For plugin development in ReadyAPI, Java is a logical choice since ReadyAPI is written in Java.
Groovy
Based on Java, but with added features, Groovy is available for use as an internal scripting language inside ReadyAPI, and some optimizations for Groovy ClassLoaders have been made in ReadyAPI.
Choosing Language
Since Groovy and Java play nice together, you don't really have to choose either one. Pick any or use them in a combination as you feel comfortable.