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 for easier navigation

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 so they remain copy-paste ready.

Test plan

• Verify language switcher links work across all README files
• Confirm code blocks and internal links render correctly
• Spot-check translation accuracy for key setup sections (Installation, Async usage, Error handling)

[PhoenixCPH]

Hi team — wanted to share a bit of context on this one.

Perplexity's API is used by developers all over the world, but the README has only been in English so far. For an SDK like this, docs are often the first thing people read before pip install, and if setup instructions or error-handling notes aren't in their language, it's easy to give up or open an issue that could've been avoided.

This PR adds full README translations in six languages (ZH, ES, FR, PT, RU, DE) with a simple switcher at the top. All code samples, env var names, and API identifiers are left as-is so nothing breaks copy-paste workflows — only the explanatory text is translated.

Happy to adjust any wording if something reads awkwardly in a particular locale. Thanks for considering!