owning language means knowing it, speaking it, using it without any hesitation. no more complication and need of litmus paper test.
I don’t find people having CNIC in Sindhi traveling in Pakistan faced any problem. My own cousins work in Islamabad having id card in Sindhi. No big issue for them. I don’t have any issue if someone opt to have id card in Sindhi / Urdu or anyother language.