chrome extension- manifest version 2 _locales issues

It sounds like you are missing the required _locales directory in your extension root. Since you are specifying a default_locale, you need to provide a _locales directory, inside of which lie your internationalization options. In your case, you would need to at least have a folder titled _locales/en, inside of which you have a messages.json file, inside of which you would set your specific parameters (see here for more information).