Phrases with msgid_plural are not translated

[okellogg]

An input such as

#: src/Gui/DelaySpinBox.qml:26 src/Gui/SmartSpinBox.cpp:30
#, kde-format
msgctxt "Integer number of seconds"
msgid " second"
msgid_plural " seconds"
msgstr[0] ""
msgstr[1] ""

is not translated, i.e. the generated output msgstr[0] and msgstr[1] are empty, just as in the input.