Is this to avoid breaking lines where they shouldn't be? If so, we
already have fill-french-nobreak-p; see fill.el.
Thanks, I didn't know that.
If this is for some other purpose, i.e. if typing French text needs to
insert no-break-space character for a purpose other than filling the
text correctly, then would someone please explain the needs, before
presenting solutions?
French text typed in Emacs is not necessary rendered in Emacs only: An HTML page written in Emacs but rendered in Firefox will have it's line breaks at different places and non-breaking spaces will apply for Firefox as well as Emacs.
Other example: I use to type text in Emacs and later copy-paste in word processing applications; I am happy to have non-breaking spaces already in place.