You seem to misunderstand. That internal "button" translation file is created by and part of Qt. We do not create it. If a Ukrainian version simply does not exist in the Qt sources, you need to contribute one to Qt.
I already rename the file manually to just uk so that the if there exists a matching internal qtbase uk one it can be loaded..
If Qt has a matching Qt_base translation file, Sigil already copies and installs it.
See:
https://github.com/Sigil-Ebook/Sigil...gil.cmake#L326
If you can not get Qt to accept your language contribution, we could then house it in our github tree but getting it to Qt would be best for all qt projects. If we add it to our tree then you or some other volunteer would need to keep it up to date.