Content Translation: Expand default supported target languages

[wprashed]

What?

Closes #981

Expands the default target languages supported by the Content Translation experiment in WordPress/ai to include 9 additional languages:

• Bengali (bn)
• Indonesian (id)
• Polish (pl-pl)
• Portuguese (Portugal) (pt-pt)
• Russian (ru-ru)
• Swedish (sv-se)
• Turkish (tr-tr)
• Ukrainian (uk)
• Vietnamese (vi)

Also updates docs/experiments/content-translation.md to list the newly supported languages.

Why?

WordPress powers websites across the globe in dozens of languages. Providing broader default language coverage out of the box makes the Content Translation experiment immediately useful to non-English sites without requiring manual filter registration.

How?

• Added the 9 languages with localized exonym labels (__( '...', 'ai' )) into Languages::get_supported_languages() in includes/Experiments/Content_Translation/Languages.php.
• Updated LanguagesTest::test_supported_languages_filter_can_add_custom_language() to use Esperanto (eo) as the custom filter test language instead of Polish (pl-pl), since Polish is now supported by default.
• Updated documentation in docs/experiments/content-translation.md to list all 23 default target languages in alphabetical order.

Use of AI Tools

AI assistance: Yes
Tool(s): Antigravity
Used for: Identifying missing languages, diagnosing test runner behavior, updating documentation, and formatting PR details.

Testing Instructions

1. Enable the Content Translation experiment in AI Settings.
2. In the Block Editor, test translating content and verify the new target languages appear in the language selection menu.
3. Verify custom languages can still be added/removed using the wpai_content_translation_languages filter.
4. Run npm run test:php or unit tests to ensure all language tests pass.

Screenshots or screencast

N/A - Non-visual backend and documentation additions.

Before	After
N/A	N/A

Changelog Entry

Added - 9 new default target languages to the Content Translation experiment.

[github-actions]

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Co-authored-by: wprashed <wprashed@git.wordpress.org>
Co-authored-by: jeffpaul <jeffpaul@git.wordpress.org>
Co-authored-by: dkotter <dkotter@git.wordpress.org>
Co-authored-by: theaminulai <theaminuldev@git.wordpress.org>
Co-authored-by: noruzzamans <noruzzaman@git.wordpress.org>

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.52%. Comparing base (f5dd5d8) to head (57250ce).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #986      +/-   ##
=============================================
+ Coverage      81.51%   81.52%   +0.01%     
  Complexity      2924     2924              
=============================================
  Files            122      122              
  Lines          11655    11664       +9     
=============================================
+ Hits            9500     9509       +9     
  Misses          2155     2155              

Flag	Coverage Δ
unit	81.52% <100.00%> (+0.01%)	⬆️

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[dkotter]

@wprashed Thanks for the contribution. As mentioned here, please ensure your PR description has all required items from our PR template.

[jeffpaul]

@estelaris for your review on languages added covering your needs ^