C#-Code

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace CountWords {
  class Program {
    static string
      filename = "",
      text = "";

    readonly static char[] alphabet = GetLowerAlphabet();

    static void Main() {
      do {
        Console.Write("Datei>");
        filename = Console.ReadLine();
      } while(!File.Exists(filename));

      //Text-Datei ist nicht ASCII-Kodiert.
      //Ist auch Unsinn da ASCII keine Umlaute beinhaltet, auf welche aber explizit eingegangen werden soll.
      text = File.ReadAllText(filename, Encoding.UTF7).ToLower();

      char[] separators = text.Where(x => !alphabet.Contains(x)).Distinct().ToArray();
      string[] words = text.Split(separators, StringSplitOptions.RemoveEmptyEntries);

      Dictionary<int, int> WordLengths = CountWordLength(words);

      foreach(var WordLength in WordLengths) {
        Console.WriteLine("{0}: {1}", WordLength.Key, WordLength.Value);
      }
      Console.WriteLine("Gesamt: {0}", WordLengths.Sum(x => x.Value));

      Console.ReadKey(true);
    }

    static char[] GetLowerAlphabet() {
      List<char> AllTheLetters = new List<char>() {
        'ä', 'ö', 'ü', 
      };

      for(char letter = 'a'; letter <= 'z'; letter++) {
        AllTheLetters.Add(letter);
      }

      return AllTheLetters.ToArray();
    }

    static Dictionary<int, int> CountWordLength(IEnumerable<string> words) {
      Dictionary<int, int> AllTheLengths = new Dictionary<int, int>();

      foreach(string word in words) {
        if(!AllTheLengths.ContainsKey(word.Length)) {
          AllTheLengths.Add(word.Length, 1);
        } else {
          AllTheLengths[word.Length]++;
        }
      }

      return AllTheLengths.OrderBy(x => x.Key).ToDictionary(x => x.Key, x => x.Value);
    }
  }
}