Why Localizations.localOf(context).toLanguageTag() returns me incorrect value for some languages?

[hello-nico-chan]

Steps to reproduce

There are not many more steps, just switch the language in app settings and print the Localizations.locale (context).toLanguageTag().toLowerCase(), then check the result.
Please check the video on my real device; the language code is at the bottom of the settings page. I tested these languages:

language	expected	actual	correct?
en-gb	en-gb	en-gb	✅
en-nz	en-nz	en-nz	✅
en-us	en-us	en-au	❌
zh-cn	zh-cn	zh-cn	✅
zh-tw	zh-tw	zh-cn	❌

Flutter 3.24.3
Dart 3.5.3
DevTools 2.37.3

Expected results

en-us and zh-tw

Actual results

en-au and zh-cn

Code sample

import 'package:flutter/material.dart';

class LanguageHelper {
  static String getLocaleString(BuildContext context) {
    return Localizations.localeOf(context).toLanguageTag().toLowerCase();
  }
}

Text('language code: ${LanguageHelper.getLocaleString(context)}'),

Screenshots or Video

ScreenRecording_01-19-2025.15-46-22_1.mov

Logs

No response

Flutter Doctor output

➜  henhaoji git:(HMA-104) ✗ flutter doctor
Doctor summary (to see all details, run flutter doctor -v):
[✓] Flutter (Channel stable, 3.24.3, on macOS 15.2 24C101 darwin-x64, locale zh-Hans-CN)
[✓] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
[✓] Xcode - develop for iOS and macOS (Xcode 16.2)
[✓] Chrome - develop for the web
[✓] Android Studio (version 2024.1)
[✓] VS Code (version 1.96.4)
[✓] Connected device (5 available)
[✓] Network resources

• No issues found!

[darshankawar]

@hello-nico-chan
Can you confirm if this occurs with native app as well or just while using flutter ?

Per https://api.flutter.dev/flutter/dart-ui/Locale/toLanguageTag.html, it returns syntactically valid unicode identifier.

[github-actions]

Without additional information, we are unfortunately not sure how to resolve this issue. We are therefore reluctantly going to close this bug for now.
If you find this problem please file a new issue with the same description, what happens, logs and the output of 'flutter doctor -v'. All system setups can be slightly different so it's always better to open new issues and reference the related ones.
Thanks for your contribution.

[github-actions]

This thread has been automatically locked since there has not been any recent activity after it was closed. If you are still experiencing a similar issue, please open a new bug, including the output of flutter doctor -v and a minimal reproduction of the issue.