a) LOCALE_INSTALL_DIR is a deprecated variable from KDEInstallDirs,
which will not be set with KDE_INSTALL_DIRS_NO_DEPRECATED
b) if using KI18n without KDEInstallDirs, but e.g. GnuInstallDirs or
custom install dir variables, having to explicitly set LOCALE_INSTALL_DIR before calling KI18N_INSTALL() is not best DX. Best is to follow typical install command macros and have this as explicit argument by the name DESTINATION, instead of (KDE-specific) automagic