You need something akin to add_definitions(-DTRANSLATION_DOMAIN=\"plasma_wallpaper_org.kde.image\") and a file such as wallpapers/image/Messages.sh · master · Plasma / Plasma Workspace · GitLab (this is handled by the kpac thing it seems).
And checkout qt qml translation documentation.
So the QT il18n can match the translations to the code.
Generally a good idea to have a look at reference implementation in Plasma/breeze, like here wallpapers/image/CMakeLists.txt · master · Plasma / Plasma Workspace · GitLab compared to plasma-smart-video-wallpaper-reborn/CMakeLists.txt at main · luisbocanegra/plasma-smart-video-wallpaper-reborn · GitHub