So still I don’t know where to download .po files
https://translate.classicpress.net/projects/core/pl/pl/ there it only opens as glotproject.
“You have to log in to add a translation.” So non logged user can’t edit.
For me - the best structure is to have all ( and only) pl_PL files in one catalog just for download. And the same for every language. That’s more easy to handle from the point of usability of single translator.