There is no /admin/config/regional/translate/import page in Drupal 9 now, which exists in Drupal 7. So I don't know how to import manually translated PO files.
You must have forgot to enable the Interface Translation core submodule, which provides Interface Translation Import at
D9 and D7 have a nearly identical UI for translation import that works the same way.
Thank you for your tip! I did enalbe the Interface Translation module. It turned out there was a CSS sinppet which hide the second secondary tab in my administration theme. :-(– VincentAug 30, 2022 at 0:35