Add translation for README to include Vietnamese.

Issue for this PR

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

This PR adds a Vietnamese translation for the main README.md (README.vi.md) to make the project more accessible to Vietnamese-speaking developers.

Additionally, I updated the language selection links at the top of all existing README.*.md files to ensure consistency across the documentation. The new Vietnamese link was appended to the end of the language list uniformly across all translated READMEs as well as the main English one.

How did you verify your code works?

Visually inspected the generated README.vi.md to ensure the markdown renders correctly and the translation is accurate. I also checked a few other README.*.md files to confirm the language links list is updated and properly aligned.

Screenshots / recordings

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

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