auto-detect doesn't work when translating from zh_CN to en

Awesome Job ! This project really helps me a lot ! But maybe there is a small issue about language auto-detect when translating from zh_CN to en.

Based on the document , if you want to translate a sentence from en to zh_CN , you just need to use three lines:

In [1]: from translate import Translator
In [2]: translator= Translator(to_lang="zh")
In [3]: translation = translator.translate("This is a pen.")
Out [3]: 这是一支笔

So based on my understanding , if you want to implement the reversed action, you also need three lines:

In [1]: from translate import Translator
In [2]: translator= Translator(to_lang="en")
In [3]: translation = translator.translate("这是一支笔")
Out [3]: 这是一支笔 # Still the original sentence. translation doesn't work

But if you specify the input language,it will work:

In [1]: from translate import Translator
In [2]: translator= Translator(from_lang="zh_CN",to_lang="en")
In [3]: translation = translator.translate("这是一支笔")
Out [3]: This is a pen # Translate successfully

Below is the screenshot tested on python3.5:

Start by reproducing the two Translator examples in the issue, then trace the Translator auto-detection path for a zh_CN source with en as the target. Done means the automatic case translates the Chinese sentence to English while the explicit from_lang case continues to work.