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](https://developer.chrome.com/docs/extensions/reference/api/i18n).