title: In computer science, what is NOT a formal language?
body: <p>On the wikipedia <a href="https://www.wikiwand.com/en/Formal_language" rel="nofollow">https://www.wikiwand.com/en/Formal_language</a>, I found the definition of a formal language:</p>
  
  <blockquote>
    <p>In mathematics, computer science, and linguistics, a formal language
    is a set of strings of symbols that may be constrained by rules that
    are specific to it.</p>
  </blockquote>
  
  <p>This looks quite abstract to me. And I can't image any language which doesn't fit to this definition. Does anyone have ideas about what a <strong>informal language</strong> looks like and how it doesn't fit the definition?</p>