You can set your fields to translate: false. That way, values cannot be translated and Kirby will fall back to the value of the default language in other languages.
Is there a way to hide a field for the default language? I was thinking of adding an info field that would advise the user to switch to the default language in order to make changes to the settings.
Maybe with a little trick: a hidden field that stores the language code as a default when a page is created, then make the info field a conditional field.