If a place is linked to a boundary and has different names, these names are added to the boundary. However, Nominatim doesn't do the expansion to the default language of the country there.

Example: the boundary of Popayan gets linked to the town node. The boundary only has an ugly descriptive name while the node has the proper town name. English speakers get the _place_name:en while for Spanish speakers the name:es is preferred. From the _place_name, the additional _place_name:es (the default language of Colombia) needs to be inferred.