Translation of the standard Qt objects is not working in 5.5 (for Portuguese language)

The translation of the standard Qt objects (e.g. the "Yes/No/Cancel" strings of message boxes) is not working in 5.5 for the Portuguese language translation file (qt_pt). It used to work in 5.2.1.

The attached Qt project example shows how the qt_pt.qm from Qt\5.5\mingw492_32\translations doesn't affect a standard message box with three options.

In version 5.2 the same project displays a correctly translated message box.

Just as an addendum, all custom strings are translated correctly (with "tr"), the problem occurs only with the standard objects from Qt library.