All gamelogic related coding is done with python.
That includes gamemodes, squad restrictions, rallypoints, logistic system and some more.
The engine itself has been written in C++ but can not be accessed and modified by any modders.
The main menu is done with flash and the vehicles, weapons, objects are coded with a special bf2 scripting language.
Some other features like weapon selection, compass and BUIS are also implemented with a combination of bf2 scripting and dirty hacks.
Becoming a dev is easy. Make something awesome like weapons, vehicles, any coding task etc and send it to us for a review

. Everyone started somewhere.
You can always apply for a spot in the testing team, but keep in mind that we're not actively looking for new testers at the moment.