Add English and Japanese documentation

[hjosugi]

Summary

• add English and Japanese counterparts for project-authored README and documentation files
• add a consistent language switcher while preserving existing document paths
• align stale or mismatched translations and add a bilingual root README where missing

Why

Public documentation should be accessible in both English and Japanese across the repository.

Validation

• verified every language-switch link resolves to an existing file
• verified translation pairs for language content, size, headings, fenced code blocks, and link targets
• checked for duplicate switchers, translation preambles, outer Markdown fences, and secret patterns
• ran git diff --check

Third-party vendored documentation was intentionally left unchanged.