📝 Update copilot-instructions.md (#732)

Author: cyfung1031

.github/copilot-instructions.md

@@ -139,3 +139,8 @@ pnpm run coverage      # Generate coverage reports
 - Tests co-located with source files (`.test.ts` suffix)
 - Template files use `.tpl` extension for build-time processing
 - Configuration files use factory pattern for environment-specific setup
+
+## Language
+- The code is developed and maintained by developers based in Mainland China.
+- Comments should preferably be written in Simplified Chinese.
+- The user interface supports multiple languages, with English set as the default for global users.