Specify language in sphinx's conf.py

docs/conf.py

@@ -48,7 +48,7 @@
 #
 # This is also used if you do content translation via gettext catalogs.
 # Usually you set "language" from the command line for these cases.
-language = None
+language = 'en'
 
 add_module_names = False
 # List of patterns, relative to source directory, that match files and