So far the best step by step tutorial I found Localizely: Flutter localization workflow streamlined - with intl pacakge, intl plugging. Also they offer some service for translating resources that worth to check out. One more good article How to Internationalize a Flutter App Using intl and intl_translation

If S.of(context) returns null, make sure you have no a MaterialApp widget inside other one MaterialApp. Also there's a problem when you try to use S.of(context) in MaterialApp widget itself, where you set up localizations.

IOS

For ios there's some additional actions required using xcode:

https://flutter.dev/docs/development/accessibility-and-localization/internationalization#appendix-updating-the-ios-app-bundle

From flutter documentation

• Flutter docs: Internationalizing Flutter apps

• Docs: Where do I store strings? How do I handle localization?

Flutter has basic support for accessibility on Android, though this feature is a work in progress.

Flutter developers are encouraged to use the intl package for internationalization and localization.