New scripting languages can be added to IGB via plugins that implement JSR-223.
To create a plugin for a scripting language, find a ScriptEngine implementation for the language, and include it in the plugin with an implementation of the ScriptProcessor interface. Then in the plugin Activator start() method, include the line:
ScriptProcessorHolder.getInstance().addScriptProcessor(new MyScriptProcessor());
Note that OSGi is not compatible with the java Service Provider framework, so the usual way of getting the ScriptEngineFactory, ScriptEngineManager.getEngineByName(name) or ScriptEngineManager.getEngineByExtension(extension) does not work. Instead, the ScriptProcessor has a method:
public ScriptEngineFactory getScriptEngineFactory();
that returns the ScriptEngineFactory. You can find the ScriptEngineFactory implementation by looking in the META-INF/services/javax.script.ScriptEngineFactory file. You may be able to just call the no parameter constructor.
There are several JSR-223 scripting language implementations at http://java.net/projects/scripting, but you can find them elsewhere.