The game engine is not written in RuneScript, but instead Java. RuneScript was developed by Jagex to allow their other staff that do not know java to make and add content to the game. The game engine then reads the RuneScript files and executes them accordingly, this structure is how most games and big programs are written. This is so that someone can change a file called 'monster1.xyz' and the main program reads that new file and is able to execute it. This system is developed for a similar reason to wiki markup, where certain code is translated to HTML ([[page]] changes to the HTML code <a href="/wiki/Page" title="Page">page</a>).
We write in a scripting language called RuneScript, which our Game Engine team has created specifically for making RuneScape content. The Java-based game engine then reads this RuneScript code and makes it all work. RuneScript is constantly being changed and expanded as we add new features to it to allow it to do new things."
Hmm, syntax wise it's similar to any C-style language, except variable names are prefixed with their type (eg. %varname is an integer). We also lack full array support. Other than that, it'd do pretty much anything you could do in C or Java.
The minor updates are usually for introducing a new map and adding new NPCs, and for these updates, a RuneScript file is added into the program. The major updates are for the source code, which is usually for making large changes such as introducing new areas in RuneScape, new skills, or for bug fixes.