How can I do a batch conversion of HTML entities to Hanzi?
You can install recode via the Terminal with Homebrew:
brew install recode

and then use it to convert HTML to Unicode, like this:
echo '&amp;#21696;&amp;#29282;&amp;#23665;' | recode html..utf8

This produces

哀牢山

(inspired by @creving's answer on Stack Overflow)