Hello Chris,
I found this video (in french) that explains how to do a pull request: https://www.youtube.com/watch?v=U5xYWy6zJGY. The advantage is that you can have your own repository and I can easily integrate your commits. The drawback is that if you're not used to it, it may not be practical to use. Especially if I also make commits on my side or integrate other pull requests because you'll have to keep your fork up to date (https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork).
If it's too complicated, I can do as I wrote above: integrate your work through a commit by tagging you. But I also thought of another solution: I can add you as a collaborator to the Github repository so that you can contribute directly.
I understand what you have set up with the different repositories for the different versions of Processwire. But I think it's better to use the tags and releases to do it. For example, on the current repo, it is possible to download .zip files of old tags: https://github.com/v-maillard/pw-lang-fr/tags and install them directly by the admin. If you want to provide zips to your friends, it would be easier to use tags.
To sum up: in my opinion, the best solution is pull requests (you can make pull requests from your repo fork). But if it's too complicated, the simplest solution is to add you as a collaborator (you'll have direct access to the repo). In any case, you'll have to work with .json files and we'll use tags to archive the versions in .zip. What solution do you prefer?
Vincent