It comes from linguist-tools, looks like you'll get it with USE="nls"
(or any of several USE cases)
alan@khamul ~ $ equery belongs /usr/lib64/qt5/bin/lrelease
* Searching for /usr/lib64/qt5/bin/lrelease ...
dev-qt/linguist-tools-5.6.1 (/usr/lib64/qt5/bin/lrelease)
alan@khamul ~ $ equery depends linguist-tools
* These packages depend on linguist-tools:
app-text/sigil-0.9.5-r1 (>=dev-qt/linguist-tools-5.4:5)
dev-db/sqlitebrowser-3.8.0 (qt5 ? dev-qt/linguist-tools:5)
kde-apps/kde-l10n-16.04.2 (>=dev-qt/linguist-tools-5.5.1:5)
kde-apps/ktp-l10n-16.04.2 (>=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/extra-cmake-modules-5.24.0 (test ?
>=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kauth-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kbookmarks-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kcodecs-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kcompletion-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kconfig-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kcoreaddons-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kcrash-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kdbusaddons-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kdesignerplugin-5.24.0 (nls ?
>=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kdnssd-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kglobalaccel-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kitemviews-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kjobwidgets-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/knotifications-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kwidgetsaddons-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/kwindowsystem-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/solid-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
kde-frameworks/sonnet-5.24.0 (nls ? >=dev-qt/linguist-tools-5.5.1:5)
x11-misc/sddm-0.13.0-r4 (dev-qt/linguist-tools:5)
Alan