OS X App bundle not signed (app "cannot" be launched)
The OS X application bundle is not signed by a developer certificate. Therefore the app cannot be opened with the default OS X settings (unless the usual trick is employed of right clicking the app bundle and selecting "Open", after which it will always open normally).
The solution is for the company (I presume CZ.NIC) to become a member of Mac Developer Program (if not already) and sign the app bundle with the developer certificate Apple automatically issues to it.
Tested with https://secure.nic.cz/files/datove_schranky/4.0.2/datovka-4.0.2-osx10.9.dmg On OS X 10.10.1