Using the :lang pseudo class selector, you can style an element based on a clients locale.

The snippet below will color the text inside of any span tag with the lang attribute ca

span:lang(ca) {  
  color: #eee;
}
<!doctype html>  
<html lang="ca">  
<head></head>  
<body>  
  <span>No n'hi ha prou amb una llengua</span>
</body>  
</html>