docs: add Bengali, Spanish, Hungarian, and Japanese CoC translations

[yunseo-kim]

Summary

This PR adds four new Contributor Covenant 3.0 Code of Conduct translations and updates cross-references across all existing CoC files.

• What changed?

  • Added Bengali (bn), Spanish (es), Hungarian (hu), and Japanese (ja) translations in docs/
  • Updated language navigation links in all existing CoC files (English, Korean, Chinese, German, French)
  • Updated the translations table in README.md

• Why is this needed?

  • Expands community accessibility by providing the Code of Conduct in additional languages spoken by contributors.

Related Issues

• Closes #
• Related #

Change Type

• Bug fix
• Feature
• Documentation
• Test/CI
• Breaking change
• Other

Checklist

General

• PR title follows Conventional Commits format: type(scope): Summary
• This PR does not expose backend/internal implementation details in a public repo.
• No secrets, tokens, keys, or private endpoints are included.
• Changes stay within this repository's intended scope.

CI/Workflow Changes (if applicable)

N/A — no workflow changes.

Protocol / Compatibility Impact

• No protocol/spec impact

Testing

• Unit tests added/updated
• Integration or conformance tests added/updated
• Tests pass
• Lint and format pass
• Type check passes
• Manual verification performed

Describe test evidence:

• All Markdown files passed markdownlint-cli2 and prettier via pre-commit hooks.
• Verified that all inter-translation links resolve correctly.

Documentation

• README updated
• Spec/docs updated
• Changelog/release notes updated (if needed)

Rollout / Risk

• Risk level: Low
• Rollback plan: Revert the three commits on this branch.

Reviewer Checklist

• Scope is clear and minimal
• Security and boundary checks passed
• Tests and docs are sufficient
• Compatibility impact is correctly handled