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. Code samples, API identifiers, environment variables, and URLs are kept unchanged.

Test plan

• Verify language switcher links work across all README files
• Confirm code blocks and external links render correctly
• Spot-check translation accuracy for installation and async usage sections

[PhoenixCPH]

Quick context on this PR.

Groq's Python SDK is used by developers worldwide, but the README has only been in English. For an API client like this, docs are usually the first stop before pip install groq — if setup, async usage, or error-handling sections aren't readable in someone's language, they're more likely to bounce or open issues that could've been avoided.

This adds six locale variants (ZH, ES, FR, PT, RU, DE) with a switcher at the top. Code samples, env var names, and API identifiers stay identical so copy-paste workflows don't break. Happy to refine any translations if something reads off.