How to use:
Visit Google Takeout
then click "Select none" and only check the box next to Hangouts.
Within a few minutes or so you will recieve a zip file with Hangouts.json inside it.
Extract the file and choose it above. if you have a very large chat history it can take a few minutes to load the file.
Don't worry, everything here is done clientside so there's no risk of your chat history being sent anywhere.
You can view the source code here
if you want to make sure. You can even just download this HTML file and run it in your browser locally.