Babelize_shell () not working

I was going through NLP using python and I encountered the following error:

babelize_shell()
NLTK Babelizer: type 'help' for a list of commands.
Babel> how long before the next flight to Alice Springs?
Babel> german
Babel> run
0> how long before the next flight to Alice Springs?
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/usr/local/lib/python2.7/dist-packages/nltk/misc/babelfish.py", line 175, in babelize_shell
    for count, new_phrase in enumerate(babelize(phrase, 'english', language)):
  File "/usr/local/lib/python2.7/dist-packages/nltk/misc/babelfish.py", line 126, in babelize
    phrase = translate(phrase, next, flip[next])
  File "/usr/local/lib/python2.7/dist-packages/nltk/misc/babelfish.py", line 106, in translate
    if not match: raise BabelfishChangedError("Can't recognize translated string.")
nltk.misc.babelfish.BabelfishChangedError: Can't recognize translated string.

      

        
        
        
      

    

I imported nltk.book as mentioned;

from nltk.book import*

      

        
        
        
      

    

* Introductory examples for the NLTK book *

Loading text1, ..., text9 and sent1, ..., sent9

Enter the name of the text or sentence to view it.

Enter: "text ()" or "sents ()" to list the materials.

text1: Moby Dick by Herman Melville 1851

text2: Sense and Sensibility Jane Austen 1811

text3: Genesis

text4: Corpus Inaugural Address

text5: Chat Corpus

text6: Monty Python and the Holy Grail

text7: Wall Street Journal

text8: Personals Corpus

text9: The Man Who Was Thursday by G.K. Chesterton 1908

can someone help me .. I am new to python and do not understand what exactly is going on.