Should you use "internal abbreviations/slang" inside comments, that is, abbreviations and slang people outside the project could have trouble understanding, for instance, using something like //NYI instead of //Not Yet Implemented?

There are advantages of this, such as there is less "code" to type (though you could use autocomplete on the abbreviations) and you can read something like NYE faster than something like Not Yet Implemented, assuming you are aware of the abbreviation and its (unabbreviated) meaning.

Myself, I would be careful with this as long as it is not a project on which I for sure will be the only developer.