I have managed to reproduce your use case in a development enviromnent using docker. I can give you the Akeeba Backup zip file of the solution to what you asked for and also the system plugin I created to solve your problem. They are already done but I don't know If I can share them publicly since it's for your client. You can contact me email@example.com so I can send you the files for this solution.
Here is a short video for the demo:
Video demo of how it works
Here is the link to the plugin source code, zipped extension and zipped backup reproductible dev environment showed in the video demo
Link to the plugin source code and the dev environment backup
Here is the overall process:
System - Compound Language Filter
Joomla! System Plugin
This plugin is based on the core languagefilter plugin and adds the feature of using multiple sef language tag
in the url associated with the same language internally.
HOW IT WORKS:
This plugin is an enhancement of the language filter plugin. It enables the possibility to have
one to many relationships from one source language to any other "virtual languages". I call them virtual languages
because they are actually not installed in Joomla!. What I did to achieve this is to leverage the power of the Joomla!
subforms and use part of the com_languages form from the admin area and add to it what I called the "source_language".
The source_language is a standard form field from Joomla! which list the installed languages of the "Frontend"
part of Joomla!.
The website I built to reproduce the use case is a multilingual website with 4 languages French, English,
Spanish and German.
In order to use the name of the country rather than the language name we can simply change the title of the
language and/or the native title accordingly. Moreover, I added the possibilty to choose the "virtual languages" and the
installed languages from the mod_languages module which Helper class has been dynamically replaced by another one by my
system plugin so as to have this custom functionnality.
By merging the subform results and the installed languages result using Joomla! Registry . It allows a powerful tool to
address this use case.
With all this you are now able to do as you describe.
NOTE: This implementation also changes the website to be multilingual for either "virtual languages" and
Just one side note:
The url of type /ar changes the active menu item to Spain and the content is in Spanish
but the menu item doesn't stay on Argentina
(I think you can get around that but I didn't have much time to answer your question)
This Joomla! plugin source code is in the src/ directory of this zip file
The final extension is in the build/ directory
If you want to build the extension yourself, make sure you have make utility on your machine.
It is generally provided by default on linux and macOS machines maybe not on Windows.
Once you have make installed.
You can type in your terminal: (you must be in the directory on the Makefile)
The Makefile rule will check if the php files has no syntax errors and create a zip file tagged by the current datetime
in the build/ directory.
If you don't want to bother doing this. Just install the already built extension from the build/ directory