What happened here seems to be simply that the Zorin OS developers have decided to include GIMP at its latest version, the much awaited 3.x, but have not yet handled all of the language packs.
This is where the complexity of maintaining software repositories is hidden away from users. Even a single piece of software may have multitude of additional packages to go along with it that also need to be kept in sync. Things like extensions, plugins or as in this case, language packs.
In that sense, rolling-release distributions make it easier by simply making the software available "as is". As always, it's all about the different pros & cons.
I would suggest re-installing GIMP, but using the Flatpak version as it will most likely contain the language packs. There may be some other issues given the sandboxed nature of Flatpak packages, but we can deal with those later.
Alternatively, you can downgrade GIMP to version 2.x, which will likely be fully compatible with language packs. To downgrade, first launch a terminal window and run the following command:
apt show gimp | grep -i version
This should return at least one entry showing the versions that are available. As I'm not on Zorin OS at the moment I don't know exactly what that would look like to you, but should look something like this:
Version: 2.10.34-1+deb12u5
Version: 2.10.34-1+deb12u3
In my case, I have two possible versions to choose from, and as it just so happens both are on version 2. Yours will probably have one in version 3 (the one you have installed) and the other on version 2. If you do not have any at version 2, then you're out of luck and should use the Flatpak version instead.
Otherwise, you can first remove the current version of Gimp:
sudo apt remove gimp
And install the other package by specifying the version like so:
sudo apt install gimp=2.10.34-1+deb12u5
Again, copy/paste the exact version that applies to you. You'll have to repeat the steps to install the French language pack, and see if this works.