fix: distinguish simplified and traditional chinese prompts

[qinxuye]

Summary

• clarify language prompts so Chinese requests preserve Simplified vs Traditional script
• update Auto and DAG response_language schema examples to use Simplified Chinese / Traditional Chinese instead of generic Chinese
• add tests covering the prompt/schema wording

Tests

• ruff check src/xagent/core/agent/language.py src/xagent/core/agent/pattern/auto/auto.py src/xagent/core/agent/pattern/dag/plan_generator.py tests/core/agent/test_context.py tests/core/agent/test_auto.py tests/core/agent/test_dag.py
• PYTHONPATH=src pytest tests/core/agent/test_context.py tests/core/agent/test_auto.py tests/core/agent/test_dag.py

[gemini-code-assist]

Code Review

This pull request updates the language policies, prompts, and tool schemas across the agent core to explicitly distinguish between Simplified Chinese and Traditional Chinese, preventing them from being collapsed into generic Chinese. Unit tests are also updated to verify these new language rules. The review feedback suggests refining the prompt in language.py to handle cases where generic Chinese is specified by falling back to the script of the user request to avoid LLM confusion.