所以我正在尝试使用 nltk 和 babelfish 学习机器翻译，但是每当我尝试使用 babelfish 方法时，我都会不断收到这些错误：
>>> import yahoo
>>> import nltk
>>> from nltk.misc import babelfish
>>> babelfish.translate('cookbook', 'english', 'spanish')
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/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.
>>> for text in babelfish.babelize('cookbook', 'english', 'spanish'):
... print text
...
cookbook
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
File "/usr/lib/python2.7/dist-packages/nltk/misc/babelfish.py", line 126, in babelize
phrase = translate(phrase, next, flip[next])
File "/usr/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.
我在这里做错了什么？