This is not a must. You can install it or not. It is used to make automatic updates to theme but it’s not necessary for theme’s normal behavior. You can either install it manually. Find the installable zip file inside theme’s folder: handel/plugins/.
You can use WPML (tested with theme, 100% compatible) or Loco (not officially tested but users have used it and works well). You can get any other translation plugin and try it out.
