Clean up localized README language switcher

[baskduf]

Goal

Improve the localized README language switcher so it is clear and consistent across English, Korean, Japanese, and Simplified Chinese README files.

Scope

• Review README.md, README.ko.md, README.ja.md, and README.zh-CN.md.
• Keep the first agent prompt block aligned across localized READMEs.
• Fix presentation issues without changing the prompt-first adoption content.
• Add or update tests only if the consistency contract changes.

Acceptance Criteria

• Language switcher text renders clearly in each README.
• python -m unittest tests.test_readme_prompt_drift passes.
• python scripts/check_encoding_hygiene.py passes.
• Docs drift and full local validation pass.

References

• ROADMAP.md
• CONTRIBUTING.md
• tests/test_readme_prompt_drift.py