l10n_sample.dart

import 'package:flutter/material.dart';

import 'l10n/app_localizations.dart';  // L10N

class AppLocale extends StatelessWidget {
  const AppLocale({ Key key, this.locale, this.child }) : super(key: key);

  final Locale locale;
  final Widget child;

  @override
  Widget build(BuildContext context) {
    return Localizations.override(
      locale: locale,
      context: context,
      child: child,
    );
  }
}

class HelloWorld extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Center(child: Text(AppLocalizations.of(context).helloWorld)); // L10N
  }
}

class App extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      localizationsDelegates: AppLocalizations.localizationsDelegates, // L10N
      supportedLocales: AppLocalizations.supportedLocales,             // L10N
      home: AppLocale(
        locale: const Locale('es'),
        child: Scaffold(
          body: Center(
            child: HelloWorld(),
          ),
        ),
      ),
    );
  }
}

void main() {
 runApp(App());
}

Can you take a look at this:

flutter/gallery#359