Hi everybody !
I finally found a way to solve my problem.
For some reason, most of the modified files in my patch.xp3 were taken into account, except for those from my scenario.xp3. To make my modified scenarios work, I had to rename them by adding a "2" at the end of each title: for example, "prologue.ks" became "prologue2.ks", and then I had to modify all the "prologue.ks" entries inside of first.ks (and other files) by "prologue2.ks" so it can lead to my translation! And it worked!
Now I just have to be careful to replace all the "prologue.ks" by "prologue2.ks". I'll have to do the same for the other modified KS files from scenario.xp3. I It's annoying, but not difficult
Again, thank you so much to KuronekoRM for trying to help me !