It worked, by using for example
I only use the import via GUI, so I did not test the CLI import.
I have the following setup:
default language: de german
I created all the levels of description in the GUI when switched to german. For every level, I changed the language and added the translation
So for example "Dossier" (de), "Fonds" (en) and "cartella" (it)
The slug of Dossier is always ...index.php/dossier no matter which language ist chosen.
Now, when importing EAD with the langusage set, it will use that particular dossier, showing "cartella" in italian, and will not create a new "cartella" with the slug ...index.php/cartella
which is what I wanted.