docs: add multi-language README support

[PhoenixCPH]

Summary

• Add README translations in Chinese, Spanish, French, Portuguese, Russian, and German
• Add a unified language switcher to all README variants

Changes

File	Language
README-ZH.md	中文
README-ES.md	Español
README-FR.md	Français
README-PT.md	Português
README-RU.md	Русский
README-DE.md	Deutsch
README.md	Updated language navigation

Each translation preserves the full structure of the English README, including installation commands, authentication notes, and legal/privacy sections. Install scripts and URLs are kept unchanged.

Test plan

• Verify language switcher links work across all README files
• Confirm install commands and external links render correctly
• Spot-check translation accuracy for authentication and data-use sections

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[PhoenixCPH]

Quick note on why I opened this.

Antigravity CLI is a terminal tool — a lot of developers will land on this repo from search, docs links, or SSH sessions where English isn't their first language. The README covers install, auth, and the data-use disclaimer; if those sections aren't readable in someone's language, they're more likely to bounce before even running the install script.

This PR adds six locale variants (ZH, ES, FR, PT, RU, DE) with a switcher at the top. Install commands, URLs, and code blocks stay identical so nothing breaks copy-paste. Happy to refine any translations if something reads off.