Hello Robin,
Sorry for this late reply, I got sidetracked on some other activities.
1) Yes, I'm working on a French application, so I selected French in the Publish properties, so the required version of the Framework4 is French.
2) Noticing that this file (french) was missing in the BootStrapper\Packages folders, I simply added it. Now, if these files are hashed, obviously this won't work.
3) No, I haven't enabled SameSite. The link you provided is indeed intended for Net Framework 3.5 and I am simply not good enough to translate that into Framework 4. This procedure includes adding references to the Product.xml file, adding a crypto key, creating a structure of folders on my hard drive with names such as "dotNetFx20", "dotNetFX30", "dotNetFX35", downloading files, using fwlinks... The documentation is entirely designed for version 3.5. I don't know when to replace the "3.5" with "4". I don't know if changing the file names will require to also change the crypto key. I don't know if I should add another folder named "dotNetFX40". I don't know if I should download the 3.5 version or if there exists a 4 version of these files. I don't know why the 3.5 documentation requires me to download one file for x86 and one file for x64, when my Framework4 is intended for both versions... Too much for me.
Besides, it is now obvious this is a Microsoft BUG (please notice that if I check the Framework4 Client as a prerequisite, there is no problem). And that procedure would be only TRYING to fix it. No real assurance it will work....
Finally, the cost of NOT doing this is very light. I simply have to UNCHECK the Prerequisites box in the Publish Properties and my problem disappears ! True, I am not able to test whether the Framework 4 is pre-installed, but the ClickOnce program stops if it isn't. And if the user complains, I can simply answer it is a Microsoft bug !