Use the [`browser.i18n`API](https://developer.chrome.com/docs/extensions/reference/i18n) to
internationalize your extension. Start by creating directories to hold
language-specific message files within a folder called `_locales/`. For example:

    _locales/en/messages.json
    _locales/es/messages.json

Each messages.json file will contain localized strings identified by a key. For
example, the following code localizes the tooltip for `_locales/en/messages.json`.
The key for this string is `__MSG_tooltip__`.

    {
      "__MSG_tooltip__": {
       "message": "Hello!",
       "description": "Tooltip"
     }
    }

You will use this key inside the manifest.json in place of the `"default_title"`.

    {
      "name": "Tab Flipper",
     ...
      "action": {
        "default_title": "__MSG_tooltip__"
      },
      "default_locale": "en"
     ...
    }

For more information, see
[Localization message formats](https://developer.chrome.com/docs/extensions/how-to/ui/localization-message-formats).