Steps to reproduce:
1. Go to the Site administration ► Language ► Language packs
2. Make sure you have no Deutsch (de*) lang pack installed
3. Install "Deutsch - Kids (de_kids)"
de_kids is installed, de_du is installed (as a parent) and de is installed, too (as a parent of de_du)
only de_kids and de_du is installed. The installer does not check for "parents of parents".