The following tutorial was generously detailed by Oliver on our forums (https://www.joomla51.com/forum/localze-web-fonts).

Currently the fonts are almost all Google Webfonts that are called directly form the Google fonts server. In the European GDPR it is recommended to localize the font files so there is no connection to a Google server, which stores personalized data like IP addresses in the US. The following steps details how this can be done within your Joomla51 template.

• I got to https://google-webfonts-helper.herokuapp.com/fonts and search the font I am using (or more than one but than one after the other). I check the styles I want to use and copy the css code. This code I store in a CSS file which I call local-fonts.css and upload it to templates/j51_xxx/css/
• Then i download the prepared ZIP file at the end of the css code section on the webfonts helper page. I unpack it and upload the font files to templates/j51_xxx/fonts/ . Now the references in the CSS file local-fonts.css point to the correct font files (e.g.: url('../fonts/gudea-v10-latin-regular.woff2') format('woff2') points to ../fonts/ which is the directory whrere I uploaded the font files)
• The last step is the most critical one, because it will be overwritten in case of a template update. I open the file templates/j51_xxx/inc/fonts.php. There I disable the lines with the loop, starting with..

  foreach ($googlefonts as $v) { // loop
  

  Below this line add...

  $app->getDocument()->addStyleSheet('/templates/j51_xxx/css/local-fonts.css');