This is indeed a complex task. We did something like this for a client one (based on a relation and a custom traverser) and are in the process of reimplementing the same feature in a different way based on how the Assets-Folder
plone.app.multilingual work. See
plone.app.multilingual.content.lif.connect_with_others for the basic idea of pointing _tree to the tree of the folder to mirror.
If it works out it should allow you to create a type 'Mirror' that mirrors the content of a folder that you can select. The content inside the mirror folder will be editable, linkable and searchable. You can have multiple mirrors to one folder and these mirrors should be able to be translatable.
Our use-case is to offer the content of a folder containing language-independent products that should be visible and searchable in different places of a multilingual website with language-specific domains.
The old implementation is still used here: https://www.dyna-jet.com/hochdruckreiniger
I will post here once it is available.