The translation system for applications based on KDE Frameworks 5 has been changed in relation of KDE4. To support additional languages in umbrello installations some changes sources and build system is required. Reproducible: Always https://blog.martin-graesslin.com/blog/2014/06/how-to-test-and-fix-translations-for-frameworks-5-applications/
If this is still an issue, please ask for help on the kde-i18n-doc mailing list. Developers there are experienced with the changes needed.
This was fixed. The tarball shipped starting with KDE Applications 17.08 (when Umbrello is officially marked as KF5 application, at least by default, waiting for the day when master is pure-KF5) contains the translations and the macros to install them.