Indeed – the Ukrainian translation of the Qt system does not exist, so it is difficult to expect it in the near future.
I can offer a workaround. I am attaching a "minimal" file, which is enough to make everything work nicely in Sigil.
There is a Russian translation inside, which will allow you to prepare a Ukrainian version.
Then from the TS file, prepare the QM file (lconvert.exe is available after installing Qt, but if you send me the TS file I'll do it right away).
Code:
lconvert.exe -i qtbase_uk_UA.ts -o qtbase_uk_UA.qm -if ts -of qm
Then just add
qtbase_uk_UA.qm file to the Sigil translations folder and it will work (file must have exactly this name, as the suffix of the language must match the suffix of the Sigil interface translation).
Unfortunately, such a prepared "minimal" file will not be added to the Sigil installer, as this would require manual intervention each time, and this, with the automation of the whole process, is not an option.
In the attachments, I attach a few more places where this translation is used:
sigil-qtbase1.png – context menu in Code View Window
sigil-qtbase2.png – hint for Close Tab
sigil-qtbase3.png – context menu for scrollbar