Whether you want to mod the game or just read the code, you'll need QGen (available in the game files since v2.0).
QSP documentation — Tip: There's a keyword section on the bottom of the menu if you're looking for a specific function.
Python — There are a few little Python scripts that might help you such as "txtsplit.py" or "MakeQSP.bat" (which uses "txtmerge.py"). It's generally a good thing to have if you're modding.
Working with Git: If you plan to use platforms like Git to host your project (or you just don't like the QSP editor), there are separate files for each location inside "game/locations". Edit them individually, make your changes and after you're done, inside the "Qgen" folder you'll find "MakeQSP.bat". Run it and execute the instructions you want.
The names of some variables are different in the code. It was originally coded in Russian, after all. Here's a partial translation table:
|In Game||In Code|
There are also some magic numbers:
They can be found in #var_base.
What files to translate by Porculus (2016-04-04)