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Wel, I think I've found out. The path with the settings I was looking for :
/home/$USER/.local/share/cura/5.7/
And in the /home/$USER/.local/share/cura/5.5/ folder, I've found the conf files that needs to be restore.
Note : creating a folder with the same name as the appimage ending with .home or .config
(e.g : ultimaker-cura-5.6.0-linux-x64.appimage.home) will get this specific folder to be used as /home or ~/.config for this appimage
mediaklan's post in Cura 5.7 linux (appimage) : some settings lost was marked as the answer