Some config items are not available in Plasma 5. At least I found none. Neither in the gui nor in kcmshell5.

a) (On Fedora 22) sometimes I have to switch application languages between two languages. I found no menu path for it. Fortunately I found a way to do this in "kcmshell4 language". And it helped, although it apparently had no effect on the registry of Plasma 5 (.config/...)

b) Similarly helped kcmshell4 to change the phonon backend from gstreamer to VLC (kcmshell4 kcm_phonon), to enable kmplayer and amarok playing mp3 files. In that context I found that there are two config files for the same task. I didn't discover a way to change .config/kde.org/libphonon.conf:

[Backends]
1\iid=org.kde.phonon.gstreamer
size=1
...

eqivalent to .kde/share/config/servicetype_profilerc:
[PhononBackend]
Entry0_Preference=2
Entry0_Service=phononbackends/vlc.desktop
Entry1_Preference=1
Entry1_Service=phononbackends/gstreamer.desktop

NumberOfEntries=2

You need the phonon4qt5 configuration panel: it shows up on my distro in System Settings, so it's either not packaged, or you may need to install additional dependencies.

I'll be able to check on a Fedora install if need be.