Delphi, C++, Pascal, Basic?

[NorWor]

In what programming language was written GDT?

[Venomous]

It was written in JavaScript.

[JayCheetah]

Announcement: First beta version with modding support is live

What language can I write mods in?

Just as the game itself, mods are created using JavaScript, HTML and CSS.

As patrick said

[apljee]

yay you used the thing he replied to me in