The graphical version of Debian Installer will use several different font sets, depending on the language.

For each new language, an OpenType font must be available in the main Debian archive. This font must respect the Debian Free Software Guidelines, in short it must be a free font.

For this step to be completed, the name of the font and the relevant Debian package must have been recorded by the Debian Installer i18n coordinators in the DebianInstallerGUIFonts wiki page (https://wiki.debian.org/DebianInstaller/GUIFonts).

The Debian Installer i18n coordinators will record this information in the supported languages list by changing the nlp_step field value to ttf_font.