Can't implement internationalization of nbextensions

I installed the nbextensions from [this direction](https://github.com/ipython-contrib/jupyter_contrib_nbextensions). At the same time, I want i18n of jupyter notebook, so I followed [this direction](https://github.com/jupyter/notebook/tree/master/notebook/i18n).
What I am puzzled is that, I tried to add Chinese infomation to nbjs.po, nbui.po and notebook.po, after compiling, the content is still in English.

I inserted these code into nbui.po and nbjs.po, found the root page didn't change. How could I change them for i18n?
```
msgid "Nbextensions"
msgstr "插件"
```

Start with the linked Jupyter Notebook i18n directions and inspect nbjs.po, nbui.po, and notebook.po, including how they are compiled. Reproduce the reported Chinese translation for “Nbextensions” and trace why the root page remains English. Done means the translated string is compiled and displayed on the root page.

Written by the indexing model from the issue text.