Exemplo de como trocar o local em uma aplicação flutter manualmente.

manual_locale.dart

import 'package:flutter_gen/gen_l10n/app_localizations.dart';
import 'package:flutter/material.dart';
import 'package:guedder/shared/util/loc.dart';

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  runApp(const MyApp());
}

class MyApp extends StatefulWidget {
  const MyApp({super.key});

  @override
  State<MyApp> createState() => _MyAppState();
}

class _MyAppState extends State<MyApp> {
  Locale localSelecionado = const Locale('pt', 'BR');

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      theme: ThemeData(colorSchemeSeed: Colors.green),
      title: 'Material App',
      home: Scaffold(
        appBar: AppBar(
          title: Builder(builder: (ctx) {
            return Text(ctx.loc.hello_title);
          }),
        ),
        body: Center(
          child: Builder(
            builder: (ctx) {
              return Column(
                mainAxisSize: MainAxisSize.min,
                children: [
                  DropdownButton<Locale>(
                    value: localSelecionado,
                    items: [
                      DropdownMenuItem(
                        value: Locale('pt'),
                        child: Text('Português'),
                      ),
                    ],
                    onChanged: (novoLocal) {
                      setState(() {
                        localSelecionado = novoLocal ?? localSelecionado;
                      });
                    },
                  ),
                  Text(ctx.loc.hello_world)
                ],
              );
            },
          ),
        ),
      ),
      localizationsDelegates: AppLocalizations.localizationsDelegates,
      supportedLocales: AppLocalizations.supportedLocales,
      locale: localSelecionado,
    );
  }
}