A mother tongue as in the Oxford English Dictionary is the language one has grown up speaking from early childhood. In other definition it is the language one learns at the knees of his mother. So, technically, Arabic is the mother tongue of the Copts. But is it really? Only a minority of ignorant Copts would say that Arabic is their national language.
As babies can be fed on artificial milk and not their mothers’ breast milk so are Copts who learn Arabic from their mothers’ and grow up speaking it: it’s only an artificial language of the Arab occupiers of Egypt, and who still hold the power of the state, and which is forced on them. It can never be in the same category of genuinely acquired matter as that of breast milk passed from the mother to her child.
Languages passed from mothers to their children can only qualify for the term ‘mother tongue’ if they are the national languages of the mother and not languages imposed on the mothers or their ancestors. Another term such as, e.g., ‘imposed mother tongue’ or ‘artificial mother tongue’ should be used in these circumstances.
It is not a trivial matter at all since it’s a matter that is strongly linked to one’s identity.
I suspect there are many nations in the same boat as ours: the Kurds in Turkey who speak Turkish from early childhood and the Amazing in Algeria who learn Arabic as babies are but only two examples.