function ErsetzeUmlauteUndEszet(const s: string; ZweiteAuchGross: boolean):  
string;  
var  
  x: integer;  
  z: char;  
  function a(c: char; i: integer): string;  
  begin  
    result := chr(ord(s[x]) - i) + c;  
  end;  
begin  
  result := '';  
  z := chr(101 - ord(ZweiteAuchGross) * 32); 
  for x := 1 to length(s) do begin  
    case s[x] of  
      'Ö', 'Ü': result := result + a(z, 135);  
      'ö', 'ü': result := result + a('e', 135);  
      'Ä': result := result + a(z, 131);  
      'ä': result := result + a('e', 131);  
      'ß': result := result + 'ss';  
    else result := result + s[x];  
    end;  
  end;  
end;  
  
// Beispielaufruf  
procedure TForm1.Button8Click(Sender: TObject);  
var  
  s: string;  
begin  
  s := 'ÄÖÜäöüß';  
  s := ErsetzeUmlauteUndEszet(s, true);  
  showmessage(s);  
end;