On my machine, there are a lot of languages/Unicode ranges that the GenericFamily::SystemUi family doesn't display because it doesn't have any fonts to fall back to. It wants to fall back to various "Noto Sans [script] UI" fonts, which I don't have installed:

Changing this to GenericFamily::SansSerif works, but this changes the preferred family as well:

Fontconfig comes with a stock config file to fall back to sans-serif for all families that are not sans-serif, serif, or monospace. This includes the other generic families like system-ui. This lets other applications (e.g. Inkscape) fall back properly:

In lieu of reimplementing all of fontconfig's matching code, we could probably just hardcode this fallback behavior here. I'm not sure how it interacts with language mapping or the "style classes", or about the right level to implement it at.