fix(i18n): protect JSON field values from language instruction in config generator

Ensure poster_type stays PascalCase English and stance stays English enum
values regardless of language setting. Only natural language fields follow
the user's language preference.

backend/app/services/simulation_config_generator.py

@@ -703,7 +703,7 @@ class SimulationConfigGenerator:
 }}"""
 
         system_prompt = "你是舆论分析专家。返回纯JSON格式。注意 poster_type 必须精确匹配可用实体类型。"
-        system_prompt = f"{system_prompt}\n\n{get_language_instruction()}"
+        system_prompt = f"{system_prompt}\n\n{get_language_instruction()}\nIMPORTANT: The 'poster_type' field value MUST be in English PascalCase exactly matching the available entity types. Only 'content', 'narrative_direction', 'hot_topics' and 'reasoning' fields should use the specified language."
 
         try:
             return self._call_llm_with_retry(prompt, system_prompt)
@@ -867,7 +867,7 @@ class SimulationConfigGenerator:
 }}"""
 
         system_prompt = "你是社交媒体行为分析专家。返回纯JSON，配置需符合中国人作息习惯。"
-        system_prompt = f"{system_prompt}\n\n{get_language_instruction()}"
+        system_prompt = f"{system_prompt}\n\n{get_language_instruction()}\nIMPORTANT: The 'stance' field value MUST be one of the English strings: 'supportive', 'opposing', 'neutral', 'observer'. All JSON field names and numeric values must remain unchanged. Only natural language text fields should use the specified language."
 
         try:
             result = self._call_llm_with_retry(prompt, system_prompt)