Initially, Godot was designed to support multiple scripting languages (this ability still exists today). However, only GDScript is in use right now. There is a little history behind this.

In the early days, the engine used the Lua scripting language. Lua is fast, but creating bindings to an object oriented system (by using fallbacks) was complex and slow and took an enormous amount of code. After some experiments with Python, it also proved difficult to embed.

The last third party scripting language that was used for shipped games was Squirrel, but it was dropped as well. At that point, it became evident that Godot would work more optimally by using a built-in scripting language...

http://docs.godotengine.org/en/latest/reference/gdscript.htm...