feat: set default lang to english; fix ext lang setting

packages/core/src/PageAgentCore.ts

@@ -417,10 +417,8 @@ export class PageAgentCore extends EventTarget {
 			return this.config.customSystemPrompt
 		}
 
-		let systemPrompt = SYSTEM_PROMPT
-
 		const targetLanguage = this.config.language === 'zh-CN' ? '中文' : 'English'
-		systemPrompt = systemPrompt.replace(
+		const systemPrompt = SYSTEM_PROMPT.replace(
 			/Default working language: \*\*.*?\*\*/,
 			`Default working language: **${targetLanguage}**`
 		)

packages/core/src/prompts/system_prompt.md

@@ -10,7 +10,7 @@ You excel at following tasks:
 </intro>
 
 <language_settings>
-- Default working language: **中文**
+- Default working language: **English**
 - Use the language that user is using. Return in user's language.
 </language_settings>