Content Translation: Expand default supported target languages

[wprashed]

Fixes #981

Description

Expands the default target languages supported by the Content Translation experiment in WordPress/ai to include additional major languages used across global WordPress sites.

Added Default 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)

All entries are localized with __( '...', 'ai' ). Custom languages remain fully extensible or filterable via the wpai_content_translation_languages filter.

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.

[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.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

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

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

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 75.39%. Comparing base (813c81d) to head (21abc4f).

Additional details and impacted files

@@              Coverage Diff              @@
##             develop     #986      +/-   ##
=============================================
+ Coverage      75.37%   75.39%   +0.01%     
  Complexity      3357     3357              
=============================================
  Files            138      138              
  Lines          12976    12985       +9     
=============================================
+ Hits            9781     9790       +9     
  Misses          3195     3195              

Flag	Coverage Δ
unit	75.39% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[dkotter]

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