title: isLetter with accented characters in C
body: <p>I'd like to create (or find) a C function to check if a char c is a letter...
  I can do this for a-z and A-Z easily of course.</p>
  
  <p>However i get an error if testing  c == á,ã,ô,ç,ë, etc</p>
  
  <p>Probably those special characters are stored in more then a char...</p>
  
  <p>I'd like to know:
  How these special characters are stored, which arguments my function needs to receive, and how to do it?
  I'd also like to know if are there any standard function that already does this.</p>