Blender integration is written in python and armory doesn’t use c/c++ for anything related to blender.
Python in blender is used for gui and for connecting to armory(again gui), c/c++ is only used when you have to develop blender feature itself, so in noway c/c++ is going to be useful for armory development with blender, doesn’t mean that c/c++ isn’t use at all in armory development, c/c++ is used in Kore, it is really less likely that you will need to touch Kore yourself.
What written in Haxe is Kha and Iron and couple big/small libraries like Zui, Armory2d(Canvas Editor), Bullet(Physics) etc.
Think of this as this way, The core working is Iron, below Iron is Kha(i.e., Iron is build on kha), Armory is merely connection between Iron and Blender.
So what Armory has is Python files that connect Blender and Iron, Shader (What armory’s rendering looks like, it has shader of voxels GI/AO, compositor, etc.) and also handle things like renderpath, and has some internal system
What Iron has is everything(It itself is engine), this include how animation works, vectors, maths, how renderpath works, handle how it own .arm format works.
What Kha has is well, this SDL but on Steroid, this handle cross-platform exporting thingie.(I think you will need to touch kha and kore less)
Depends on what type of missing feature you want to implement.
These language and things you will basically need to learn: Haxe, Python and Blender’s Api, Glsl shading, json parsing in haxe(not in top list but it is used in some areas)
But like @MikeRobinson is trying to say, it not gonna be easy trying to contribute, you must continue to learn and learn, so the very first step is completely understand how Armory works and its structure
(I Might be wrong somewhere)