title: check if char isletter
body: <p>i want to check if a string only contains correct letters.
  I used <code>Char.IsLetter</code> for this.
  My problem is, when there are chars like é or á they are also said to be correct letters, which shouldn't be.</p>
  
  <p>is there a possibility to check a char as a correct letter A-Z or a-z without special-letters like á?</p>