docs: add multilingual README translations

[shinpr]

Summary

• add Simplified Chinese, Japanese, Spanish, Korean, and Brazilian Portuguese README translations
• add a language selector to every README variant
• refine localized technical terminology based on language review while preserving the English source meaning

Verification

• confirmed matching heading, code fence, details block, and table structure across all README variants
• verified recipe names, plugin names, agent names, and external URLs against the English README
• validated every local Markdown target
• ran git diff --cached --check

This is a documentation-only change, so no runtime tests were required. The local commit hook reported that lefthook was not installed; the documentation checks above were run directly.