Comment 7 for bug 1844653

I confirmed that the Silesian language can be selected in and installed via Language Support.

Silesian should also be added to localechooser's languagelist, so it can be chosen in the installer. I failed to find a repo which matches the package; this is probably the closest:

https://code.launchpad.net/~ubuntu-core-dev/localechooser/+git/localechooser

Anyway, attached please find a debdiff instead with a proposed oneliner.