KCM fonts panel under Plasma systemsettings5 is broken on nvidia based systems. The RVB submenu and hinting menu have empty previsualisation and settings can't be changed. This is fixed in a later version of Plasma Workspace.
Fixed upstream in https://bugs.kde.org/show_bug.cgi?id=408606 (Fixed in Plasma Workspace 5.21)
URL: (none) => https://bugs.kde.org/show_bug.cgi?id=408606
This is fixed in Cauldron. The commit could be backported for Mageia 8.