Translation zh-cn (#781)

* add zh-CN README

* add language selector for zh-CN and ENG

Author: Blaire0228

README.md

@@ -1,3 +1,5 @@
+🌍 [English](/README.md) | [简体中文](./docs/i18n/README.zh-CN.md)
+
 ![Agent Governance Toolkit](docs/assets/readme-banner.svg)
 
 # Welcome to Agent Governance Toolkit!
@@ -81,7 +83,7 @@ pip install agentmesh-lightning        # RL training governance
 - **[Threat Model](docs/THREAT_MODEL.md)** — Trust boundaries, attack surfaces, and STRIDE analysis
 - **[Architecture](docs/ARCHITECTURE.md)** — System design, security model, trust scoring
 - **[Architecture Decisions](docs/adr/README.md)** — ADR log for key identity, runtime, and policy choices
-- **[NIST RFI Mapping](docs/nist-rfi-mapping.md)** — Mapping to NIST AI Agent Security RFI (2026-00206)
+- **[NIST RFI Mapping](docs/nist-rfi-mapping.md)** — Mapping to NIST AI Agent Security RFI (2026-00206)
 
 Still have questions? File a [GitHub issue](https://github.com/microsoft/agent-governance-toolkit/issues) or see our [Community page](COMMUNITY.md).