Thanks, that was a good help. It does not come from a language file. It is hardcoded in
plugins/finder/k2/k2.php (2 times) and will then have to be renamed in the database tables
__finder_taxonomy
__finder_types
Old and new indexes will be added to the new term. Unfortunately only for this translation now so no solution for multilanguage sites. Hopefully K2 changed this in the upcoming J4 version.