docs: add multi-language output guide

docs/multi-language-output.md

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+# Multi-Language Output Guide
+
+Configure OpenSpec to generate artifacts in languages other than English.
+
+## Overview
+
+OpenSpec can output proposals, specs, designs, and tasks in any language by adding language instructions to your project's `context` configuration. This approach uses the existing config system without requiring any code changes.
+
+## Quick Setup
+
+### If you already have a config file
+
+Add a language instruction to your existing `openspec/config.yaml`:
+
+```yaml
+schema: spec-driven
+
+context: |
+  Language: Portuguese (pt-BR)
+  All artifacts must be written in Brazilian Portuguese.
+  Use Portuguese technical terminology where appropriate.
+
+  # Your other project context below...
+  Tech stack: TypeScript, React, Node.js
+```
+
+That's it. All generated artifacts will now be in Portuguese.
+
+### If you don't have a config file yet
+
+You can create one interactively or manually:
+
+**Option 1: Interactive setup**
+
+```bash
+openspec artifact-experimental-setup
+```
+
+This will guide you through creating `openspec/config.yaml` with schema selection, context, and rules.
+
+**Option 2: Manual creation**
+
+Create the file `openspec/config.yaml` in your project root:
+
+```bash
+mkdir -p openspec
+cat > openspec/config.yaml << 'EOF'
+schema: spec-driven
+
+context: |
+  Language: Portuguese (pt-BR)
+  All artifacts must be written in Brazilian Portuguese.
+EOF
+```
+
+## Language Examples
+
+### Portuguese (Brazil)
+
+```yaml
+context: |
+  Language: Portuguese (pt-BR)
+  All artifacts (proposals, specs, designs, tasks) must be written in Brazilian Portuguese.
+  Use Brazilian Portuguese conventions for technical terminology.
+```
+
+### Spanish
+
+```yaml
+context: |
+  Idioma: Español
+  Todos los artefactos deben escribirse en español.
+  Utilizar terminología técnica en español cuando sea posible.
+```
+
+### Chinese (Simplified)
+
+```yaml
+context: |
+  语言：中文（简体）
+  所有产出物（提案、规格、设计、任务）必须用简体中文撰写。
+  技术术语可以保留英文原文，但说明应使用中文。
+```
+
+### Japanese
+
+```yaml
+context: |
+  言語：日本語
+  すべての成果物は日本語で作成してください。
+  技術用語は必要に応じて英語を併記可能です。
+```
+
+### French
+
+```yaml
+context: |
+  Langue : Français
+  Tous les artefacts doivent être rédigés en français.
+  Utiliser la terminologie technique française lorsque possible.
+```
+
+### German
+
+```yaml
+context: |
+  Sprache: Deutsch
+  Alle Artefakte müssen auf Deutsch verfasst werden.
+  Technische Fachbegriffe können auf Englisch beibehalten werden.
+```
+
+## How It Works
+
+The `context` field in `openspec/config.yaml` is injected into every artifact's instructions as an XML block:
+
+```xml
+<context>
+Language: Portuguese (pt-BR)
+All artifacts must be written in Brazilian Portuguese.
+...
+</context>
+
+<template>
+[Schema's template content]
+</template>
+```
+
+The AI assistant sees this context when generating any artifact and follows the language instruction.
+
+## Tips
+
+### Be Explicit About Scope
+
+Specify which parts should be in the target language:
+
+```yaml
+context: |
+  Language: Spanish
+  Write all prose, descriptions, and explanations in Spanish.
+  Code comments may remain in English for consistency with the codebase.
+  Variable names and code identifiers should stay in English.
+```
+
+### Handle Technical Terms
+
+Decide how to handle technical terminology:
+
+```yaml
+context: |
+  Language: Japanese
+  Write in Japanese, but:
+  - Keep technical terms like "API", "REST", "GraphQL" in English
+  - Provide Japanese explanations in parentheses for complex terms on first use
+  - Code examples and file paths remain in English
+```
+
+### Combine with Other Context
+
+Language settings work alongside your other project context:
+
+```yaml
+schema: spec-driven
+
+context: |
+  Language: Portuguese (pt-BR)
+  All artifacts must be written in Brazilian Portuguese.
+
+  Tech stack: TypeScript, React 18, Node.js 20
+  Database: PostgreSQL with Prisma ORM
+  Testing: Vitest + React Testing Library
+
+  Conventions:
+  - Use functional components with hooks
+  - Follow existing patterns in src/components/
+```
+
+## Verification
+
+To verify your language config is working:
+
+```bash
+# Create a test change
+openspec new change test-language
+
+# Check the instructions - should show your language context
+openspec instructions proposal --change test-language
+
+# Output will include:
+# <context>
+# Language: Portuguese (pt-BR)
+# All artifacts must be written in Brazilian Portuguese.
+# ...
+# </context>
+```
+
+## Limitations
+
+- Language instructions apply to **all** artifacts. You cannot set different languages for different artifact types.
+- The effectiveness depends on the AI model's proficiency in the target language.
+- Technical diagrams, code samples, and file paths typically remain in English regardless of language setting.
+
+## Future Considerations
+
+We're considering adding a dedicated `language` field to the config schema in a future release:
+
+```yaml
+# Potential future syntax (not yet implemented)
+schema: spec-driven
+language: pt-BR
+```
+
+For now, the `context` approach described above is the recommended method. It provides full flexibility and works with all current versions of OpenSpec.
+
+## Related Documentation
+
+- [Project Config Demo](./project-config-demo.md) - Overview of project configuration
+- [Experimental Workflow Guide](./experimental-workflow.md) - Full workflow documentation