fix incorrect language setting in sphinx documentation build

doc/UsersGuide/source/conf.py

@@ -73,7 +73,7 @@
 #
 # 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'
 
 # List of patterns, relative to source directory, that match files and
 # directories to ignore when looking for source files.