Below is a possible solution to your problem but I am unable to post the file you need to do it. It is only a text file but it is far too many characters to post here as text and I cannot post a link to it. Apple removes links (security reasons I suppose). I'll keep trying but, while you wait, you can read through these instructions to see if you want to attempt it.
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You are talking about the dot on the numeric keypad, correct? The built-in AZERTY keyboard mapping has that as a comma (unless you use Shift), as you have rightly pointed out. But you can make your own keyboard layout with that key as a dot and install the layout. There is at least one app that will do this for you (I think) but I have never used it. You can do it without the app.
Everything I say from here on you can follow at your own risk. I am no expert on keyboard layouts. I found it to be an interesting problem so I took a shot at solving it. Maybe wait until someone more experienced in this area says it is okay and my instructions are correct? Or maybe they'll recommend against this.
I downloaded an azerty layout from GitHub, remapped every instance of the "." keypad key, and installed the layout on my computer. I do not have an azerty keyboard to test it on to see if all the other key mappings are correct but it did make the "." work as a ".". If you totally do not like it, you can uninstall it and delete the file from your computer. Be aware that it may still show up as a choice in System Preferences->Keyboard->Input Sources even after it is deleted. If someone knows what file to edit to remove it from that list, I hope they post it here.
Instructions:
- Copy all the text for the file [from where I do not yet know].
- Open the TextEdit app and create a new document
- Format the document as plain text (in the format menu)
- Paste the text into the blank document
- Save as Azertydot.keylayout in your user folder /Library/Keyboard Layouts.
- Make sure the filename does not have ".txt" on the end of it. If it does, delete that part.
- Log out then log back in. Do this before going to System Preferences to install the new layout.
The rest of the instructions are how to add it as a keyboard layout. You probably already know how to do that.
- Open System Preference->Keyboard->Input Sources. Click to add a new layout. On the left side scroll to the bottom to "other". It should be in that folder.
- Add it
- Check the box at the bottom of the window for "Show Input Menu in menu bar". An icon will show up on the main menu bar at the top right where you can select which keyboard to use. Use that icon to select this new keyboard.

When you try to use it in an app, like Numbers, if it switches back to another keyboard layout, something is wrong.
If any keys are mapped differently than you expect, you can change them by editing the file. I won't get into that right now, just letting you know it can be done.
Here is the text for the file:
[I cannot put it here. It exceeds the 5000 character limit by quite a lot]