I’m trying to make a Proxmox template using Packer and I've been struggling to make it choose the French Language.
Here's a portion of my user-data file :
autoinstall:
version: 1
locale: fr_FR
keyboard:
layout: fr
I went through packer docs and couldn't find any piece of documentation about that user-data file :/ Anyone knows where I can get this information please ?

Packer its self doesnt know or care what a user-data file is. That is something thats OS specific for example like in ubuntu https://ubuntu.com/server/docs/install/autoinstall-reference#user-data. Where as other OS like windows or RHEL might have a different method to setup a system. Using the ubuntu example the way I have done this previously is with a setup like below.
In my case I am using vsphere but the idea should work on other sources like AWS EBS etc. I also have a setup.sh script I run as part of the build to configure a few other things after the system is bootstrapped
user-data
setup.sh
template.pkr.hcl