See attached doc for detailed scenario. We’re using KB category picker (treeview) for 2 fields, it works well for all languages except French.

The category picker should display instead of the listView, this is working as per the design and there is no OOB functionality you can switch on/off to display or not the category picker. Even in French language, it should display and this is working as expected
The fact it is not actually showing in "your instance" when you switch the language to french, is a different issue that could be solved by
changing the following message
https://yourInstance.service-now.com/nav_to.do?uri=sys_ui_message.do?sys_id=d03f4667dbe22b000a8ffd471d961962

from: "Rechercher à l'aide du sélecteur"
to "Rechercher à l\'aide du sélecteur"

After that, it will display in French as well.
But in this case, you want to disable the category picker. Just be aware this is an implementation or customization and it's out of the scope of the support team. As previously mentioned the category picker is controlled by the UI macro "kb_category_reference_lookup".