docs: add multi-language README support

[PhoenixCPH]

Summary

• Add README translations in Spanish, French, Portuguese, Russian, and German under doc/
• Extend the language switcher in README.md and doc/README-CN.md to link all localized README files

Changes

File	Language
doc/README-ES.md	Español
doc/README-FR.md	Français
doc/README-PT.md	Português
doc/README-RU.md	Русский
doc/README-DE.md	Deutsch
README.md / doc/README-CN.md	Updated language navigation

Each new translation follows the full structure of the English README, including Quick Start, Features, Contributing, and Development sections. Download links, commands, and technical identifiers are kept unchanged.

Test plan

• Verify language switcher links work from root and doc README files
• Confirm image paths and download links render correctly in each translation
• Spot-check translation accuracy for setup and contribution sections

[coderabbitai]

Important

Review skipped

Auto reviews are disabled on this repository. Please check the settings in the CodeRabbit UI or the .coderabbit.yaml file in this repository. To trigger a single review, invoke the @coderabbitai review command.

⚙️ Run configuration

Configuration used: Repository UI

Review profile: CHILL

Plan: Pro

Run ID: 1b6eedd5-cc0a-4ffc-a5ab-6c4233bc53be

You can disable this status message by setting the reviews.review_status to false in the CodeRabbit configuration file.

Use the checkbox below for a quick retry:

• 🔍 Trigger review

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}

[PhoenixCPH]

Hey — a bit of context on this PR.

Chatbox already ships with multilingual UI (EN, ZH, JA, KO, FR, DE, RU, ES, etc.), but the GitHub README only had English and 简体中文. That's a mismatch: a lot of users find the project on GitHub first, and if they can't read the docs in their language, they're less likely to download, set up providers, or contribute translations back.

This PR adds five more README locales under doc/ (ES, FR, PT, RU, DE) and wires up a switcher on every variant. Same layout as the English README — download tables, features, dev setup — so nothing gets lost in translation except the prose itself.

Open to feedback on wording in any locale. Thanks for taking a look!