Thanks for the tip! Here’s what I have done so far.
pkcon won’t find package jolla-xt9. It did find jolla-keyboard-settings-xt9, which I installed just in case
After that, thinking there must be something wrong indeed with my package index, I did a pkcon refresh, hoping that I would be then able to install xt9, either from the command line or from the app store, but nope. It was of no use.