I found a new kobo desktop update here
. This site looks very official.
Now i have the kobo desktop version 3.1.1 and found a portuguese dictionary inside the installation file, which I could use, but can not use because I do not speak portuguese. I copied the related three files to .kobo/dict and it works.
I tried to download this dictionary like eg. this ending .../dictionaries/dicthtml-de.zip, replacing de
, but it doesn't work. Maybe later!