Or maybe I’m only partially upgraded? Is there a more elaborate / deep down version file somewhere (via LuCi or even CLI); e.g. update-logs?
Luckily @cynerd states in that same post the issue to be nearly harmless, so maybe I should just ignore it for now (if I understand him correctly it is automatically resolved after a reboot)?
Bug is in postinst script. So when old version of package is going to be removed than it does this. New version should not do it any more. You can try to select additional language again. Wait until it’s installed and than deselect it again.
And yes it’s harmless. Single thing that that script does is that it restart lighttpd, so if you want to be on the safe side just restart lighttpd or whole router. But it’s not necessary.