Thanks for your comments. What I can answer right away is that I've found the issue 59975 and the related commit too, but I still look at it as a bug, even if the earlier behavior was accidental (i.e. if the font with Cyrillic support was chosen for its typographic characters). If a user during the installation selects their language and, after booting, sees question marks instead of all localized strings, that affects them more than seeing only ASCII progress bars.
I think the installer should conditionally revert the progress bars (is that possible?) and set LatGrkCyr-8x16 as the console font, if the system locale is Ukrainian, Bulgarian, Macedonian, Belarusian, Serbian or Russian. That's what Debian-based distributions do; other approaches are probably possible (choose the default font in the GNU service at boot time?). When I learn more of the system internals, I'll try to make a patch. Adding console-font-service-type to the Guix config like the documentation suggests didn't work, it complained that the service already existed, so I ran setfont manually to get a minimally working system with which I can work on my documents and debug the graphical display issue later. Note to self: learn about modify-services.
