Defines the default language of an extension that supports multiple locales. It is the name of the subdirectory in _locales that contains the default language for this extension. For example, the following code indicates that English is the default language:
"default_locale": "en"
This field is required for localized extensions (those with a _locales directory), but must be absent in
extensions that have no _locales directory. For details, see Internationalization.