Let say a multi-language website such as dictionary, and it supports en-us and de, and es.
Now I would indicate my website to google that all these languages are supported.
For example, if user search either World or Welt(german) or Mundo(spanish), google should find out the site useful.
In other word, I would have 3 metas tag as site description in all languages supported. something like following.
<META NAME="DESCRIPTION" CONTENT="World: where we are living">
<META NAME="DESCRIPTION" CONTENT="Welt: wo wir leben.">
<META NAME="DESCRIPTION" CONTENT="Mundo: donde estamos viviendo.">
Then user would find it if he searches the world in many languages.