I believe that when an object has been localized like say SiteTree etc, it checks the current ‘locale’ from the FluentState Class - silverstripe-fluent/FluentState.php at master · tractorcow-farm/silverstripe-fluent · GitHub
Based off of that class when you call write, publish, etc, whatever the current locale is set to is what will get modified. So If you need to update just some other locale than the default, you would probably need to call something like -
Then do the publish/write. I haven’t tested this but that is where I would start. Now if your site is configured with proper fallbacks, then you should just need to do the normal write/publish and other locales will then fall back to that.