Internet Health Report uses the Gulp Starter build system. A comprehensive explanation of the tasks and configuration settings can be found in that project's Readme.
Make sure Node is installed. This has been tested on Node 6.6.0.
npm installnpm start
This runs the default gulp task, which starts compiling, watching, and live updating files as they are changed. BrowserSync will start a server on port 3000. You'll be able to see live changes in all connected browsers.
Localised text content is managed in individual content.properties files within a language specific directory:
src/html/_locales/en-us
The HTML task compiles these with .html markup files to build seperate language directories.