Sigil in some places uses standard interface elements that are, as it were, 'ready' as soon as Qt is installed.
Imagine either using a ready-made "Default Save button" or writing a "Custom Save button" yourself.
These defaults are unfortunately translated directly in Qt and are just found in the qtbase_XX files.
An explanation perhaps clumsy, but hopefully understandable.
Personally, I had already suggested a few years ago that these dozen or so standard texts should be added to the main Sigil translation, but - as far as I remember - they did not want to multiply entities.
Edit: Joining qt_base files is also something very strange. Fort Firewall, for example, does it in the same way. Adds qt_base for languages for which there are translations.
https://github.com/tnodir/fort/tree/master/src/ui/i18n