Thanks for the explanation and the images.
In my old Macbook with an old, non-retina display, the 'App settings' submenu was way too low, outside of the visible screen, and hidden from plain view. Only when I scrolled down, I could then (easily) see the 'App settings' submenu and change the language setting.
Now everything is shown in the right language pack.
Still, I wonder why the Moodle Desktop app did not choose my preferred Moodle language pack. There are 31 language packs to choose from
Many thanks to Ralf and Juan.