Apparently this is missing from the default kdeglobals used by KDE4:

[General]
desktopFont=Noto Sans,10,-1,5,50,0,0,0,0,0
fixed=Hack,9,-1,5,50,0,0,0,0,0
font=Noto Sans,10,-1,5,50,0,0,0,0,0
menuFont=Noto Sans,10,-1,5,50,0,0,0,0,0
smallestReadableFont=Noto Sans,8,-1,5,50,0,0,0,0,0
taskbarFont=Noto Sans,10,-1,5,50,0,0,0,0,0
toolBarFont=Noto Sans,9,-1,5,50,0,0,0,0,0

Currently it uses the hardcoded default, which is "Sans Serif" and does not fit
with Plasma 5 at all.

Or is there a different mechanism to use the fonts used by Plasma 5 which is
somehow not working here?