Configure zh_CN language for sphinx-proof Chinese support

Set language to zh_CN in sphinx config to enable Chinese translations
for proof, theorem, lemma, and other sphinx-proof directives.
Also fix duplicate intersphinx_mapping key in config.

Author: mmcky

lectures/_config.yml

@@ -34,6 +34,7 @@ latex:
 sphinx:
   extra_extensions: [sphinx_multitoc_numbering, sphinxext.rediraffe, sphinx_tojupyter, sphinxcontrib.youtube, sphinx.ext.todo, sphinx_exercise, sphinx_proof, sphinx_togglebutton, sphinx.ext.intersphinx, sphinx_reredirects]
   config:
+    language: zh_CN
     # false-positive links
     linkcheck_ignore: ['https://online.stat.psu.edu/stat415/book/export/html/834']
     bibtex_reference_style: author_year
@@ -90,7 +91,6 @@ sphinx:
         google_analytics_id: G-J0SMYR4SG3
       launch_buttons:
         colab_url                 : https://colab.research.google.com
-    intersphinx_mapping: 
     intersphinx_mapping:
       intermediate: 
         - https://python.quantecon.org/