Since this issue still exists in Dorico 4 (even after these years), I am going to send a bug report ticket to Qt to ask them to supply these missing QtBase translation files.
Update 1: Ticket sent: [QTBUG-110828] Please supply QtBase translation files for all languages supported by Qt. - Qt Bug Tracker …
Update 2: I’ll see a chance to supply this file to Qt through a pull request.