Repository: flutter
Issue: Adding an unsupported locale to Flutter is challenging due to NumberFormat and DateFormat symbols and patterns
Labels: framework,f: material design,a: internationalization,has reproducible steps,found in release: 3.3,found in release: 3.7,team-design,triaged-design

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I'm trying to add a new language to my app (Nynorsk, language code = "nn").
I've followed the Flutter documentation found here: https://flutter.dev/docs/development/accessibility-and-localization/internationalization#adding-support-for-a-new-language

Since the language I'm adding isn't supported GlobalMaterialLocalizations I've also created a new GlobalMaterialLocalizations delegate like the example provided by Flutter doc (see main.dart below). This is where the problems begin.
Since Nynorsk (nn) isn't supported by Intl the calls to intl.DateFormat in the load function causes a crash.

## Steps to Reproduce

<!-- You must include full steps to reproduce so that we can reproduce the problem. -->

1. Run `flutter create bug`.
2. Update the main file as follows: 

<details>
  <summary>main.dart</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
      exception, please see if the error message includes enough information
      to explain how to solve the issue.
-->

```
import 'dart:async';

import 'package:intl/intl.dart' as intl;
import 'package:intl/date_symbol_data_local.dart' as intl;
import 'package:flutter/foundation.dart';
import 'package:flutter/material.dart';
import 'package:flutter_localizations/flutter_localizations.dart';

class _NnMaterialLocalizationsDelegate
    extends LocalizationsDelegate<MaterialLocalizations> {
  const _NnMaterialLocalizationsDelegate();

  @override
  bool isSupported(Locale locale) => locale.languageCode == 'nn';

  @override
  Future<MaterialLocalizations> load(Locale locale) async {
    final String localeName = intl.Intl.canonicalizedLocale("nn");
    await intl.initializeDateFormatting(localeName, null);
    return SynchronousFuture<MaterialLocalizations>(
      NnMaterialLocalizations(
        localeName: localeName,
        fullYearFormat: intl.DateFormat('y', localeName),
        compactDateFormat: intl.DateFormat('yMd', localeName),
        shortDateFormat: intl.DateFormat('yMMMd', localeName),
        mediumDateFormat: intl.DateFormat('EEE, MMM d', localeName),
        longDateFormat: intl.DateFormat('EEEE, MMMM d, y', localeName),
        yearMonthFormat: intl.DateFormat('MMMM y', localeName),
        shortMonthDayFormat: intl.DateFormat('MMM d'),
        decimalFormat: intl.NumberFormat('#,##0.###', localeName),
        twoDigitZeroPaddedFormat: intl.NumberFormat('00', localeName),
      ),
    );
  }

  @override
  bool shouldReload(_NnMaterialLocalizationsDelegate old) => false;
}

class NnMaterialLocalizations extends GlobalMaterialLocalizations {
  const NnMaterialLocalizations({
    String localeName = 'nn',
    @required intl.DateFormat fullYearFormat,
    @required intl.DateFormat compactDateFormat,
    @required intl.DateFormat shortDateFormat,
    @required intl.DateFormat mediumDateFormat,
    @required intl.DateFormat longDateFormat,
    @required intl.DateFormat yearMonthFormat,
    @required intl.DateFormat shortMonthDayFormat,
    @required intl.NumberFormat decimalFormat,
    @required intl.NumberFormat twoDigitZeroPaddedFormat,
  }) : super(
          localeName: localeName,
          fullYearFormat: fullYearFormat,
          compactDateFormat: compactDateFormat,
          shortDateFormat: shortDateFormat,
          mediumDateFormat: mediumDateFormat,
          longDateFormat: longDateFormat,
          yearMonthFormat: yearMonthFormat,
          shortMonthDayFormat: shortMonthDayFormat,
          decimalFormat: decimalFormat,
          twoDigitZeroPaddedFormat: twoDigitZeroPaddedFormat,
        );

  @override
  String get moreButtonTooltip => r'More';

  @override
  String get aboutListTileTitleRaw => r'About $applicationName';

  @override
  String get alertDialogLabel => r'Alert';

  @override
  String get anteMeridiemAbbreviation => r'AM';

  @override
  String get backButtonTooltip => r'Back';

  @override
  String get cancelButtonLabel => r'CANCEL';

  @override
  String get closeButtonLabel => r'CLOSE';

  @override
  String get closeButtonTooltip => r'Close';

  @override
  String get collapsedIconTapHint => r'Expand';

  @override
  String get continueButtonLabel => r'CONTINUE';

  @override
  String get copyButtonLabel => r'COPY';

  @override
  String get cutButtonLabel => r'CUT';

  @override
  String get deleteButtonTooltip => r'Delete';

  @override
  String get dialogLabel => r'Dialog';

  @override
  String get drawerLabel => r'Navigation menu';

  @override
  String get expandedIconTapHint => r'Collapse';

  @override
  String get hideAccountsLabel => r'Hide accounts';

  @override
  String get licensesPageTitle => r'Licenses';

  @override
  String get modalBarrierDismissLabel => r'Dismiss';

  @override
  String get nextMonthTooltip => r'Next month';

  @override
  String get nextPageTooltip => r'Next page';

  @override
  String get okButtonLabel => r'OK';

  @override
  String get openAppDrawerTooltip => r'Open navigation menu';

  @override
  String get pageRowsInfoTitleRaw => r'$firstRow–$lastRow of $rowCount';

  @override
  String get pageRowsInfoTitleApproximateRaw =>
      r'$firstRow–$lastRow of about $rowCount';

  @override
  String get pasteButtonLabel => r'PASTE';

  @override
  String get popupMenuLabel => r'Popup menu';

  @override
  String get postMeridiemAbbreviation => r'PM';

  @override
  String get previousMonthTooltip => r'Previous month';

  @override
  String get previousPageTooltip => r'Previous page';

  @override
  String get refreshIndicatorSemanticLabel => r'Refresh';

  @override
  String get remainingTextFieldCharacterCountFew => null;

  @override
  String get remainingTextFieldCharacterCountMany => null;

  @override
  String get remainingTextFieldCharacterCountOne => r'1 character remaining';

  @override
  String get remainingTextFieldCharacterCountOther =>
      r'$remainingCount characters remaining';

  @override
  String get remainingTextFieldCharacterCountTwo => null;

  @override
  String get remainingTextFieldCharacterCountZero => r'No characters remaining';

  @override
  String get reorderItemDown => r'Move down';

  @override
  String get reorderItemLeft => r'Move left';

  @override
  String get reorderItemRight => r'Move right';

  @override
  String get reorderItemToEnd => r'Move to the end';

  @override
  String get reorderItemToStart => r'Move to the start';

  @override
  String get reorderItemUp => r'Move up';

  @override
  String get rowsPerPageTitle => r'Rows per page:';

  @override
  ScriptCategory get scriptCategory => ScriptCategory.englishLike;

  @override
  String get searchFieldLabel => r'Search';

  @override
  String get selectAllButtonLabel => r'SELECT ALL';

  @override
  String get selectedRowCountTitleFew => null;

  @override
  String get selectedRowCountTitleMany => null;

  @override
  String get selectedRowCountTitleOne => r'1 item selected';

  @override
  String get selectedRowCountTitleOther => r'$selectedRowCount items selected';

  @override
  String get selectedRowCountTitleTwo => null;

  @override
  String get selectedRowCountTitleZero => r'No items selected';

  @override
  String get showAccountsLabel => r'Show accounts';

  @override
  String get showMenuTooltip => r'Show menu';

  @override
  String get signedInLabel => r'Signed in';

  @override
  String get tabLabelRaw => r'Tab $tabIndex of $tabCount';

  @override
  TimeOfDayFormat get timeOfDayFormatRaw => TimeOfDayFormat.h_colon_mm_space_a;

  @override
  String get timePickerHourModeAnnouncement => r'Select hours';

  @override
  String get timePickerMinuteModeAnnouncement => r'Select minutes';

  @override
  String get viewLicensesButtonLabel => r'VIEW LICENSES';

  @override
  List<String> get narrowWeekdays =>
      const <String>['S', 'M', 'T', 'W', 'T', 'F', 'S'];

  @override
  int get firstDayOfWeekIndex => 0;

  static const LocalizationsDelegate<MaterialLocalizations> delegate =
      _NnMaterialLocalizationsDelegate();

  @override
  String get calendarModeButtonLabel => r'Switch to calendar';

  @override
  String get dateHelpText => r'mm/dd/yyyy';

  @override
  String get dateInputLabel => r'Enter Date';

  @override
  String get dateOutOfRangeLabel => r'Out of range.';

  @override
  String get datePickerHelpText => r'SELECT DATE';

  @override
  String get dateRangeEndDateSemanticLabelRaw => r'End date $fullDate';

  @override
  String get dateRangeEndLabel => r'End Date';

  @override
  String get dateRangePickerHelpText => 'SELECT RANGE';

  @override
  String get dateRangeStartDateSemanticLabelRaw => 'Start date \$fullDate';

  @override
  String get dateRangeStartLabel => 'Start Date';

  @override
  String get dateSeparator => '/';

  @override
  String get dialModeButtonLabel => 'Switch to dial picker mode';

  @override
  String get inputDateModeButtonLabel => 'Switch to input';

  @override
  String get inputTimeModeButtonLabel => 'Switch to text input mode';

  @override
  String get invalidDateFormatLabel => 'Invalid format.';

  @override
  String get invalidDateRangeLabel => 'Invalid range.';

  @override
  String get invalidTimeLabel => 'Enter a valid time';

  @override
  String get licensesPackageDetailTextOther => '\$licenseCount licenses';

  @override
  String get saveButtonLabel => 'SAVE';

  @override
  String get selectYearSemanticsLabel => 'Select year';

  @override
  String get timePickerDialHelpText => 'SELECT TIME';

  @override
  String get timePickerHourLabel => 'Hour';

  @override
  String get timePickerInputHelpText => 'ENTER TIME';

  @override
  String get timePickerMinuteLabel => 'Minute';

  @override
  String get unspecifiedDate => 'Date';

  @override
  String get unspecifiedDateRange => 'Date Range';
}

class Home extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      body: Center(
        child: RaisedButton(
          onPressed: () {
            // This would fail if localizations were not provided for the
            // app's locale. Although only a few Material widgets display
            // localized strings, many expect to be able to find localized
            // values for tooltips and other strings related to accessibility.
            showDialog(
              context: context,
              builder: (BuildContext context) =>
                  Dialog(child: Center(child: Text('Hello World'))),
            );
          },
          child: const Text('Show Dialog'),
        ),
      ),
    );
  }
}

void main() {
  runApp(
    MaterialApp(
      localizationsDelegates: [
        GlobalWidgetsLocalizations.delegate,
        GlobalMaterialLocalizations.delegate,
        NnMaterialLocalizations.delegate,
      ],
      supportedLocales: [const Locale('nn')],
      home: Home(),
    ),
  );
}
```

</details>

3. 
**Expected results:** <!-- what did you want to see? -->
Expected closest "related" and supported Locale to be used (nb_NO in this case) if specified locale isn't supported, or just default locale (en_US).

Or an error message with info about how to add the missing DateSymbols and date time patterns for unsupported locale.


**Actual results:** <!-- what did you see? -->
Crash. Because Intl.verifiedLocale throws ArgumentError("Invalid locale \"nn\"")

<details>
  <summary>Logs</summary>

<!--
      Run your application with `flutter run --verbose` and attach all the
      log output below between the lines with the backticks. If there is an
      exception, please see if the error message includes enough information
      to explain how to solve the issue.
-->

```
[VERBOSE-2:ui_dart_state.cc(166)] Unhandled Exception: Invalid argument(s): Invalid locale "nn"
#0      Intl._throwLocaleError (package:intl/intl.dart:245:5)
#1      Intl.verifiedLocale (package:intl/intl.dart:239:21)
#2      new DateFormat (package:intl/src/intl/date_format.dart:249:20)
#3      _NnMaterialLocalizationsDelegate.load (package:mypackage/resources/nn_material_localization.dart:23:30)
<asynchronous suspension>
#4      _loadAll (package:flutter/src/widgets/localizations.dart:60:49)
#5      _LocalizationsState.load (package:flutter/src/widgets/localizations.dart:515:62)
#6      _LocalizationsState.initState (package:flutter/src/widgets/localizations.dart:480:5)
#7      StatefulElement._firstBuild (package:flutter/src/widgets/framework.dart:4684:58)
#8      ComponentElement.mount (package:flutter/src/widgets/framework.dart:4520:5)
#9      Element.inflateWidget (package:flutter/src/widgets/framework.dart:3490:14)
#10     Element.updateChild (package:flutter/src/widgets/framework.dart:3258:<…>
```

<!--
     Run `flutter analyze` and attach any output of that command below.
     If there are any analysis errors, try resolving them before filing this issue.
-->
Flutter analyse output:
```
No issues found! (ran in 3.9s)
```

<!-- Finally, paste the output of running `flutter doctor -v` here. -->
Flutter doctor -v output
```
[✓] Flutter (Channel stable, 1.20.4, on Mac OS X 10.15.6 19G2021, locale en-NO)
    • Flutter version 1.20.4 at /PATH_TO/flutter
    • Framework revision fba99f6cf9 (10 days ago), 2020-09-14 15:32:52 -0700
    • Engine revision d1bc06f032
    • Dart version 2.9.2

 
[✓] Android toolchain - develop for Android devices (Android SDK version 30.0.0)
    • Android SDK at /PATH_TO/Library/Android/sdk
    • Platform android-30, build-tools 30.0.0
    • Java binary at: /Applications/Android Studio.app/Contents/jre/jdk/Contents/Home/bin/java
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)
    • All Android licenses accepted.

[✓] Xcode - develop for iOS and macOS (Xcode 11.7)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Xcode 11.7, Build version 11E801a
    • CocoaPods version 1.9.3

[✓] Android Studio (version 4.0)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin version 49.0.2
    • Dart plugin version 193.7547
    • Java version OpenJDK Runtime Environment (build 1.8.0_242-release-1644-b3-6222593)

[✓] VS Code (version 1.46.1)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.12.1

[✓] Connected device (2 available)
    • sdk gphone x86 64 (mobile) • emulator-5554                        • android-x64 • Android 11 (API 30) (emulator)
    • iPhone 8 (mobile)          • 77BEAB8F-9C9E-4E72-AE5F-D7784AD94A11 • ios         •
      com.apple.CoreSimulator.SimRuntime.iOS-13-7 (simulator)

• No issues found!
```

</details>