I've confirmed that this fixes the problem. (As usual I had to preseed oem-config/install-language-support to true, and it didn't actually manage to install the language packs because they were uninstallable in the archive at the time; but it did at least try, and it displayed the dialog after login saying that language support was incomplete, so I consider that a success.)
I've confirmed that this fixes the problem. (As usual I had to preseed oem-config/ install- language- support to true, and it didn't actually manage to install the language packs because they were uninstallable in the archive at the time; but it did at least try, and it displayed the dialog after login saying that language support was incomplete, so I consider that a success.)