Thank you gmayor,
good thing to add and error management, but unexpectedly the script (put in "this document" part of visualbasic) is not launched when hitting ctrl+s (save) !?
I was working on it since this morning and still nothing interesting discovered...
EDIT :looking on the www it looks like it comes from the fact that I have no clue how different module types work...
EDIT 2: thanx to https://stackoverflow.com/a/40673321/9704680 it is working
With Word 2016 I found that a change was necessary
Set X.App = Word.Application
should be
Set X.appWord = Word.Application