Fix wp_dropdown_languages en_US selection with explicit_option_en_us

[Adi-ty]

This PR fixes a bug in wp_dropdown_languages() where the English (United States) option would not be properly selected when using explicit_option_en_us=true and selected='en_US'.

Trac ticket: https://core.trac.wordpress.org/ticket/58284

---

Tests

Added test coverage for the specific bug scenario

npm run test:php -- tests/phpunit/tests/l10n.php --filter test_wp_dropdown_languages

• test_wp_dropdown_languages_explicit_option_en_us_selected() - Tests the specific bug scenario where explicit_option_en_us=true and selected='en_US'

---

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.

[github-actions]

Test using WordPress Playground

The changes in this pull request can previewed and tested using a WordPress Playground instance.

WordPress Playground is an experimental project that creates a full WordPress instance entirely within the browser.

Some things to be aware of

• The Plugin and Theme Directories cannot be accessed within Playground.
• All changes will be lost when closing a tab with a Playground instance.
• All changes will be lost when refreshing the page.
• A fresh instance is created each time the link below is clicked.
• Every time this pull request is updated, a new ZIP file containing all changes is created. If changes are not reflected in the Playground instance,
  it's possible that the most recent build failed, or has not completed. Check the list of workflow runs to be sure.

For more details about these limitations and more, check out the Limitations page in the WordPress Playground documentation.

Test this pull request with WordPress Playground.

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

Core Committers: Use this line as a base for the props when committing in SVN:

Props iamadisingh.

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.