Fix #6009

Test Code

import flet as ft


def main(page: ft.Page):
    page.horizontal_alignment = ft.CrossAxisAlignment.CENTER

    page.add(
        ft.Button(
            content="Pick time (zh_Hans locale)",
            icon=ft.Icons.CALENDAR_MONTH,
            on_click=lambda e: page.show_dialog(
                ft.TimePicker(locale=ft.Locale("zh", "Hans"))
            ),
        )
    )


ft.run(main)

Summary by Sourcery

Add per-control locale overrides to date and time pickers and tighten locale configuration/validation across Flutter and Python layers.

New Features:

• Introduce an optional locale property for CupertinoDatePicker, DatePicker, DateRangePicker, and TimePicker controls to allow overriding page/system locale per picker instance.

Enhancements:

• Refine Python Locale and LocaleConfiguration types with stronger defaults, validation, and detailed documentation for language/region/script handling and resolution behaviour.
• Filter supported locales on the Flutter side to only include those supported by Flutter localization delegates and add a Locale.isSupportedByDelegates helper.
• Refactor multiple Dart enum parsing helpers to share a common generic parseEnum utility.
• Export the new Dart enums utility module for broader reuse.

Documentation:

• Add example apps demonstrating custom locales for CupertinoDatePicker, DatePicker, DateRangePicker, and TimePicker.

Tests:

• Extend and restructure integration tests for CupertinoDatePicker, DatePicker, DateRangePicker, and TimePicker to cover locale overrides via screenshots and per-test app fixtures.