Technically you can execute this on a device with the correct architecture {aarch64|armv7hl|i486} (i.e. aarch64 targetting the Jolla C2) and SailfishOS 4.5.0 installed (4.6.0 may also work, and is preferable then, because it provides slightly newer packages; you should yield at least the package versions listed below, which are from 4.4.0 on aarch64), to preserve the XT9 RPM packages for reinstallation later (note that this is a single command line!):
mkdir -p xt9-rpms; for i in $(pkcon -p search name xt9 | tr -s ' ' | cut -s -f 2 -d ' ' | fgrep '.jolla.' | rev | cut -s -f 3- -d '-' | rev; echo jolla-handwriting); do pkcon download xt9-rpms $i; sleep 1; done
If you wonder, why this is performed in such a complicated way: PackageKit including its command-line front-end pkcon is a piece of shit, because it loves to fail when tasked with more than one thing at a time and when commanded in quick succession. Hence the for loop, downloading one package at a time and waiting a second between pkcon commands. Also there might be RPM repositories subscribed which offer XT9-related Patches, hence the filtering.
This should download (at least) these packages (as an example the XT9 packages for SailfishOS 4.4.0 on aarch64):
feature-xt9-0.0.7-1.3.2.jolla.aarch64.rpm
jolla-handwriting-0.1.5-1.3.2.jolla.aarch64.rpm
jolla-keyboard-settings-xt9-0.8.28.3-1.7.2.jolla.aarch64.rpm
jolla-xt9-0.5.19-1.5.2.jolla.aarch64.rpm
jolla-xt9-cp-0.2.12-1.3.2.jolla.aarch64.rpm
patterns-sailfish-xt9-0.0.7-1.3.2.jolla.aarch64.rpm
# Only on aarch64:
jolla-xt9-server-0.5.19-1.4.61.jolla.aarch64.rpm
You can check that you indeed have downloaded all packages from the XT9 repository by comparing the downloaded packages (ls -1 xt9-rpms/) with the output of zypper pa -r xt9 (thanks @explit).
P.S.: BTW, Neuntrainer is a nice tool to fill your XT9-dictionary.
Edit: This executes the job more elegantly when you have zypper installed (by pkcon install zypper) and does not require checking as it is guaranteed to download all packages from the XT9 repository (but not jolla-keyboard-settings-xt9, because it is from a different repository):
mkdir -p xt9-rpms; for i in $(zypper pa -r xt9 | fgrep xt9 | cut -s -f 3 -d '|'); do pkcon download xt9-rpms $i; sleep 1; done