> Why teach C++? Some students might not prefer that to C#.

Between these two there is very little to choose in terms of concepts the students would be exposed to. I would assume that teaching either of those would aim to teach the underlying concepts of object orientated programming.

The questions that need to be asked instead are: What concepts would building an editor plugin expose students too? And is this the best way to introduce those concepts? (NB I am assuming University level where contact time for taught material is only really enough to introduce concepts not to really go into details.)

I would argue that a programming course might want to teach all three of those. They are quite different languages which could be used to teach different underlying concepts (C for pointers and low level stuff (and maybe combined with some assembler), Java for object orientation and Python for dynamic typing).

Python vs PHP vs JavaScript would be another matter. I would say it doesn't matter so much which of those you teach. Same with e.g. Java vs C# vs C++.

The question remains why build an Atom plugin over an Excel macro? And what underlying concepts would you hope to teach students by doing this? And is this the best way to introduce those concepts?

I assume that the OP is teaching at University level. From experience I know that time is very limited in that environment. Everything done with the students should be well justified. Building an editor add on seems like something I would question strongly. What concepts is the OP aiming to introduce the students to? And is this the best way to introduce those concepts?