flutter-quill/doc/configurations/localizations_setup.md

Localizations Setup

in addition to the required delegates mentioned above in Using custom app widget

which are:

localizationsDelegates: const [
    DefaultCupertinoLocalizations.delegate,
    DefaultMaterialLocalizations.delegate,
    DefaultWidgetsLocalizations.delegate,
],

which are used by official Flutter widgets

The library also needs the

// Required localizations delegates ...
FlutterQuillLocalizations.delegate

To offer the default localizations.

But you don't have to since we have wrapped the QuillEditor and QuillToolbar with FlutterQuillLocalizationsWidget which will check if it sets then it will go, if not, then it will be provided only for them, so it's not really required, but if you are overriding the localizationsDelegates you could also add the FlutterQuillLocalizations.delegate which won't change anything

There are additional notes in the Translation section