Thanks very much for the answer
I checked
Overriding icons, templates, et cetera
In this documentation, the section "Overriding icons, templates, et cetera" that says <<calibre allows you to override the static resources, like icons, javascript and templates for the metadata jacket, catalogs, etc. with customized versions that you like. All static resources are stored in the resources sub-folder of the calibre install location. >>
I was hoping that one of you would have pointed like: "Go and put that file here, modify this line in the file and you are good to go".
But if you send me to change the source (which to me amounts to a mountain to climb), it means there it is not in the static resources, and is no easy way to change it on an installed copy. Is-there?
What about forking the French localization file, and tell calibre do use 'french-tycho' instead of French. Would that do the trick? Would it remain in case of upgrade?