Add internationalization (i18n) support for PHPMailer error messages

[Sukhendu2002]

Trac ticket: https://core.trac.wordpress.org/ticket/23311

---

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 sukhendu2002, swissspidy, audrasjb.

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

[swissspidy]

Could we add a test that actually loads a translation file for those new strings and then triggers a PHPMailer error, so that we can verify the error messages are actually translated?

[swissspidy]

Let me know if you'd need help with those btw

[audrasjb]

I agree with all the changes proposed by @swissspidy 👍

[swissspidy]

@Sukhendu2002 I took the liberty to push some changes, simplifying the locale switching in the test.

When switching locales we need to ensure that the global PHPMailer instance's translations are updated. This should be achieved in WP_Locale_Switcher, not by resetting the instance in the test.