flutter-quill/doc/configurations/using_custom_app_widget.md

Using Custom App Widget

This project use some adaptive widgets like AdaptiveTextSelectionToolbar which require the following delegates:

1. Default Material Localizations delegate
2. Default Cupertino Localizations delegate
3. Defualt Widgets Localizations delegate

You don't need to include those since there are defined by default but if you are using Custom app or you are overriding the localizationsDelegates in the App widget then please make sure it's including those:

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

And you might need more depending on your use case, for example if you are using custom localizations for your app, using custom app widget like FluentApp from [FluentUI] which will also need

localizationsDelegates: const [
    // Required localizations delegates ...
    FluentLocalizations.delegate,
    AppLocalizations.delegate,
],

Note: In the latest versions of FluentApp you no longer need to add the localizationsDelegates but this is just an example, for more info

There are additonal notes in Localizations page