The projects in this book have all been written using GDScript. GDScript has a number of advantages that make it the best choice for building your games. It is very tightly integrated with Godot's API, and its Python-style syntax makes it useful for rapid development while also being very beginner-friendly.
It's not the only option, however. Godot also supports two other "official" scripting languages, and also provides tools for integrating code using a variety of other languages.