Okay so I've found what is going wrong. I've followed this conversation to delete a translation (
https://groups.google.com/g/ica-atom-users/c/55GENtGjp-Q/m/51uMq65MBQAJ) while this is working nicely for archival descriptions it does not if the slug refers to an archive. My guess is that some foreign key still links to the deleted Spanish entry and that's why the indexing is failing.
So what I did is: dump all data and "grepped" for culture "es" which can only be found in "property_i18n" . Does this makes sense to clear those DELETE * from property_i18n pin WHERE culture = "es"
What is property_i18n used for?
(By the way, this is all a test VM. So no danger to break anything ;)