Any reason to use C++?
You can create things really quick with GDScript to see if work first (is a OOP based language too so should not be a problem to use), then make modules on C++ with exactly the same logic.
People use to do this on critical parts of the games, where max performance is needed.
More or less like in Unreal.
Check GDNative post, Godot 3 will be able to use the engine API with almost any language (there are people working with Rust, Lua and others) but I would not recommend to use C++ (and I love { } ) or any languages without knowing the engine first.
Godot 3 will support C# too (not via Native scripts) but GDScript will stay as the main language.