When compiling Qt on Linux for crosscompiling to Windows, the IMPORTED_LOCATION of Qt5::lrelease is set to [...]/lrelease.exe. But this tool is built for the host system when cross compiling so the suffix .exe is wrong. The same problem exists for Qt5::lupdate.
The issue is caused in linguist.pro by the line
win32:CMAKE_BIN_SUFFIX = ".exe"
I guess this line can be removed (but I cannot test in a native windows environment) because mkspecs/features/create_cmake.prf has the correct condition for host tools:
equals(QMAKE_HOST.os, Windows): CMAKE_BIN_SUFFIX = ".exe"
(Replacing the condition in linguist.pro does solve the problem.)