flutter-quill/doc/configurations/using_custom_app_widget.md

Using Custom App Widget

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

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

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

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

You might need more depending on your use case, for example, if you are using custom localizations for your app, using a 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 additional notes on the Localizations page