title: How/Where does JavaScript detect the default locale?
body: <p>Today, the 11th of September, 2017, JavaScript's <code>toLocaleDateString()</code> method is outputting <code>9/11/2017</code> for me. But I am in the UK, so the formatting is wrong in this case. The <a href="https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Date/toLocaleDateString" rel="noreferrer">MDN Web Docs</a> tell me that this method returns "a formatted string in the default locale".</p>
  
  <p>So, where/how is JavaScript detecting my default locale? Where is this set, or what does it depend on, and (how) can I change it?</p>
  
  <p>Edited to add: I'm using Firefox 55.0.3 on Windows 10 if that makes any difference.</p>