The above step-by-step answer is clear; and it also worked for me on Kubuntu 18.04. thanks!
I was however not 100% content with the solution for two reasons:
(1) As a self-styled 'linux evangelist to the common man', I like to use - and promote - solutions that don't require use of the command line.
I have an answer to this:
1.1: add the languages using the GUI: system-settings-> Personalisation:Regional Settings->languge... Copy required languages to right panel, click on 'Apply', log out and in.
This in itself isn't enough though. You also need to...
1.2: start the software manager: Applications->System->Discover. Search for and install 'Language Support', then launch it (you can do this from 'Discover'). When prompted to add missing components, click on 'OK'.
(2) I'm reluctant to add components which use a gnome GUI to what is essentially a KDE system. I have found that this can give problems when doing routine updates later. (Admittedly, problems are only likely if you make a complete hotch-potch of KDE and Gnome features!) Note that step 1.2 above also installs a GTK front-end, so is also not ideal.
I will re-work this post if I find a cleaner solution soon!