General Settings: Show language default label on date/time format options

[sanketio]

Summary

Adds a "(Language default)" label next to the date and time format option that matches the site's language on the General Settings screen, making it clear which format the active language expects.

Problem

When a site uses a non-English language, the pre-selected date/time format strings (e.g. F j, Y) are translated by WordPress core for that language. Previously, there was no visual indicator showing which radio option represented the language's natural default, leaving admins to guess.

A prior fix (changeset 60942) was reverted before WP 6.9 because it:

1. Resolved the translated format using the user locale instead of the site locale, producing a wrong label when the two differed.
2. Concatenated the label directly into the format string, breaking RTL layouts.

Solution

Before rendering the format list, the code now:

1. Calls get_locale() to get the site locale (not the user locale).
2. If the user is browsing in a different locale, temporarily switches to the site locale via switch_to_locale() / restore_previous_locale() to resolve the translated format strings.
3. If switch_to_locale() returns false (locale already active, or language pack not installed), no label is shown — safe fallback, no label on a wrong format.

The label is rendered as a separate <span class="description"> element, outside the .format-i18n span, so existing JavaScript that reads the format preview text is completely unaffected.

Changes

File	Description
src/wp-admin/options-general.php	Locale-discovery block + conditional label in date and time format loops
tests/phpunit/tests/admin/optionsGeneral.php	New PHPUnit test class (8 tests) covering locale-discovery logic and label output
tests/e2e/specs/general-settings-date-time-format.test.js	New Playwright E2E tests (5 tests) verifying label visibility and HTML structure in-browser

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Trac ticket: https://core.trac.wordpress.org/ticket/64102

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Use of AI Tools

AI assistance: Yes
Tool(s): GitHub Copilot, ChatGPT
Model(s): GPT-5.1
Used for: Initial code skeleton and test suggestions; final implementation and tests were reviewed and edited by me.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

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Props sanketparmar.

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