I just want to make a small clarification because I think it was not explicitly said:
The Google API key is required to get the up to date list of fonts from Google in the admin panel as well as all the available variants of these fonts and their popularity. That key is not used in the front end. When you add a font its data is saved to the forum settings and is used to inject the Google Font CSS tag in the forum page. You could theoratically remove the key once you installed the fonts you wanted, this will only prevent you from accessing the "Fonts" page of the admin panel.
Google don't just let you access their API endpoints without saying who you are ?