Here's my quick answer.

jD is programmed in java, a compiled programming language. Getting started as a JD-Developer.

There are some places where scripting languages are used. One of them is the reconnection function. The reconnection recorder uses CURL, but you can also write your own batch script in any language you like.

jD has some APIs like the remote control interface, the web interface and the command interface. You can all use them to control jD's operation.

Note that there exist other search engines than Gogol's.