feat: add i18n language toggle and bilingual docs

CONTRIBUTING.en.md

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+# Contributing Guide
+
+Language: [中文](CONTRIBUTING.md) | [English](CONTRIBUTING.en.md)
+
+Thank you for contributing to DS2API!
+
+## Development Setup
+
+### Backend
+
+```bash
+# 1. Clone the repo
+git clone https://github.com/CJackHwang/ds2api.git
+cd ds2api
+
+# 2. Create a virtual environment (recommended)
+python -m venv venv
+source venv/bin/activate  # Windows: venv\Scripts\activate
+
+# 3. Install dependencies
+pip install -r requirements.txt
+
+# 4. Configure
+cp config.example.json config.json
+# Edit config.json
+
+# 5. Run
+python dev.py
+```
+
+### Frontend (WebUI)
+
+```bash
+cd webui
+npm install
+npm run dev
+```
+
+WebUI language packs live in `webui/src/locales/`. Add new locale JSON files there.
+
+## Code Standards
+
+- **Python**: Follow PEP 8, use 4-space indentation
+- **JavaScript/React**: Use 4-space indentation and function components
+- **Commit messages**: Use semantic prefixes (e.g. `feat:`, `fix:`, `docs:`)
+
+## Submitting a PR
+
+1. Fork this repo
+2. Create a feature branch (`git checkout -b feature/xxx`)
+3. Commit your changes (`git commit -m 'feat: add xxx'`)
+4. Push your branch (`git push origin feature/xxx`)
+5. Open a Pull Request
+
+## WebUI Build
+
+> **Important**: After modifying `webui/`, **no manual build is required**.
+
+When a PR is merged into `main`, GitHub Actions will automatically:
+1. Build the WebUI
+2. Commit build artifacts to `static/admin/`
+
+If you need a local build (for testing):
+```bash
+./scripts/build-webui.sh
+```
+
+## Project Structure
+
+```
+ds2api/
+├── app.py              # FastAPI entrypoint
+├── dev.py              # Development server
+├── core/               # Core modules
+│   ├── auth.py         # Account auth & rotation
+│   ├── config.py       # Configuration management
+│   ├── deepseek.py     # DeepSeek API calls
+│   ├── models.py       # Model definitions
+│   ├── pow.py          # PoW calculations
+│   └── sse_parser.py   # SSE parsing
+├── routes/             # API routes
+│   ├── openai.py       # OpenAI-compatible endpoints
+│   ├── claude.py       # Claude-compatible endpoints
+│   ├── home.py         # Landing page routes
+│   └── admin/          # Admin endpoints
+├── webui/              # React WebUI source
+├── static/admin/       # WebUI build output (auto-generated)
+└── scripts/            # Helper scripts
+```
+
+## Reporting Issues
+
+- Use [GitHub Issues](https://github.com/CJackHwang/ds2api/issues)
+- Provide detailed reproduction steps and logs