Not for all objects the data was already in the [old_name:de], for majority of them you added it, so where does this data come from? Arcanum maps? What is the license of the data there?

There are no German names for Polish suburbs. These are historical names and we cannot add historical names here in the name:de tag. They can be added in the old_name:de. Big cities or some popular tourism attractions do have German names - so the big German cities have Polish names.
You are new to OSM so please read https://wiki.openstreetmap.org/wiki/Good_practice#Don't_map_historic_events_and_historic_features and https://wiki.openstreetmap.org/wiki/Verifiability and add historical names to https://wiki.openstreetmap.org/wiki/Open_Historical_Map