peopleinside this might be because it's a ref translation. If you directly edit the original fof-nightmode.admin.settings.modal.theme_auto translation key does it work?
I suspect it's related to the fallback mechanism of Flarum coupled with references. It seems like nightmode isn't translated at all in Italian, so the original key fof-nightmode.admin.settings.modal.theme_auto does not contain the reference => fof-nightmode.ref.auto in Italian, so it means it falls back to the English value, which is => fof-nightmode.ref.auto, which points to the English text.
If you set fof-nightmode.admin.settings.modal.theme_auto: => fof-nightmode.ref.auto in Italian in Linguist, then the ref value will probably be picked up in Italian as well.
Basically you will have to enter all the => reference keys from the original file https://github.com/FriendsOfFlarum/nightmode/blob/master/resources/locale/en.yml in Linguist as well if you want the reference to read the Italian version. You can take the yml file, remove all lines without => and use the new import feature to Italian to import those values in Linguist.
