right-to-left (RTL) in flutter

you have two choices :

1. force a locale ( and direction ) on all devices

— method 1: with localization

add flutter_localizations package to your pubspec.yml

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter

then

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp(
  localizationsDelegates: [
    GlobalCupertinoLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
  ],
  supportedLocales: [
    Locale("fa", "IR"), // OR Locale('ar', 'AE') OR Other RTL locales
  ],
  locale: Locale("fa", "IR") // OR Locale('ar', 'AE') OR Other RTL locales,
  .
  .
  .
);

— method 2: without localization

MaterialApp(
  .
  .
  .
  builder: (context, child) {
    return Directionality(
      textDirection: TextDirection.rtl,
      child: child,
    );
  },
  .
  .
  .
);

2. set layout direction according to device locale
( if user phone locale is a RTL language and exist in supportedLocales, your app run in RTL mode, otherwise your app is LTR )

add flutter_localizations package to your pubspec.yml

dependencies:
  flutter:
    sdk: flutter
  flutter_localizations:
    sdk: flutter

then

import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

MaterialApp(
  localizationsDelegates: [
    GlobalCupertinoLocalizations.delegate,
    GlobalMaterialLocalizations.delegate,
    GlobalWidgetsLocalizations.delegate,
  ],
  supportedLocales: [
    Locale("en", "US"),
    Locale("fa", "IR"), // OR Locale('ar', 'AE') OR Other RTL locales
  ],
  .
  .
  .
);

note : rtl languages in flutter are:

[
  'ar', // Arabic
  'fa', // Farsi
  'he', // Hebrew
  'ps', // Pashto
  'ur', // Urdu
];