Qt 5.5 - won't use show some Qt translations

[Will]

I'm moving an existing app from Qt 5.2 to 5.5. It is localized into several different languages.

At startup I load my own localized .qm translation files, as well as the Qt UI files (e.g. qt_xx.qm). QTranslator loads and installs them without error.

However for some languages I don't see translations for basic things like "Save", "Discard", "Close" buttons in a standard QMessageBox.

German and Russian are fine, but Spanish and Chinese don't show the Qt translations. I have tied both building the .qm files into my app's resources, and as separate /languages/ folder. Both with the same results.

However on the Mac version I do see the translations in my app's menu on Mac for things like "Hide app" - so I know at least the translations are loaded.

Anyone have any ideas on this?

i know for some languages the translation files (such as "qt_it.qm") are empty and the translations are in "qtbase_xx.qm". I'm using those when available. Also tried using the windeployqt to copy the qm files for me.