@davidfor : ok, i checked the first book i bought from the kobo store
https://www.dropbox.com/s/2t5orhocyr...38.59.png?dl=0
https://www.dropbox.com/s/2vslmvwpk5...43.59.png?dl=0
Then, i remove the drm and check inside it : there's only a "justified space" (aka : a common space) after each dash. So I put fine space, and recheck : there is a difference but very slight at this police size :
https://www.dropbox.com/s/1st8pqdj05...04.54.png?dl=0
But when i change the margin and the size you can see it clearly :
https://www.dropbox.com/s/jdi8semo31...12.37.png?dl=0
The problem is that kobo doesn't use a fixed width space where it should be in french even if they conserve the correct width in the kobo application desktop.