My set defaults form view doesn't even show up like that. It does nothing. It shows a drop down field with no options and there is nothing to save. On the other hand I even deactivated English (US) but the system keeps showing up in English (US). I am on odoo 16
Thanks for the effort though.
For setting default language for the newly creating contacts, just activate the developer mode and open the contacts form view, and select the required language inside the field, and click the Debugger button in the top right of the screen near logged in user form and Click Set Defaults.
From the Default selection field, select the language, tick For all Users and Click Save Default.