Intl 패키지 사용

dependencies:
  flutter:
    sdk: flutter
  intl: ^0.16.1

DateFormat 날짜를 일본어로 설정

import 'package:flutter/material.dart';
import 'package:intl/date_symbol_data_local.dart'; // 追加
import './transaction.dart';
import 'package:intl/intl.dart'; // 追加

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {

    // 初期化のためのメソッドを追加
    initializeDateFormatting('ja');

    return MaterialApp(
      title: 'Flutter App',
      home: MyHomePage(),
    );
  }
}

class MyHomePage extends StatelessWidget {
      // 省略
@override
  Widget build(BuildContext context) {
   return Scaffold(
       appBar: AppBar(
         title: Text('Flutter App'),
       ),
       body: Column(
         // 省略
           Text(
             DateFormat.yMMMd('ja').format(tx.date),
             style: TextStyle(color: Colors.grey),
           )
        // 省略
      )
   }
}

DateFormat.yMMMd('ja').format(tx.date),