The RSTA code completion doesn't support any languages directly; rather, it is a framework through which you can add completion support for any language. As you probably discovered, on the project page
there is a Webstart example of code completion for the C standard library - that is implemented by reading all of the data from a simple XML file.
I also have a half-baked example of Java code completion, that offers completion for classes read from a user-specified "classpath", and even parses the current editor's contents for method and field names. This is an attempt at ensuring the feature is robust enough to support "dynamic" completion. One user emailed me an example he created of using the code completion feature for Groovy code completion, and is really quite good.
So in short, it's really more of a "framework" than an implementation for any specific language. I could certainly add language completion "implementations" to the project downloads, such as the C XML file in the demo, and the Java completion when (if) it's done.
Another thing I'm considering is adding support for parsing ctags output
. This would allow for good code completion if you were okay with adding a dependency on the Exuberant ctags library (it's GPL, whereas RSTA is LGPL).