Yes, if it was the colonialists doing it (and I'd think, if they were going to mess around with their subject's language preferences, they'd seek to promote English instead!) it would backfire. But what if it were a radical nationalist movement?
Iranians widely know Arabic but don't prefer to speak it; they have their own languages, as do many other Muslims who consider themselves quite faithful yet know Arabic only as a second language, if at all. The last Shah did not find it necessary to impose the Farsi language because it had simply survived (and indeed prospered, becoming a widely-known second, or third, language throughout the Muslim world, a language of poetry and culture). But he did seek to de-emphasize Iran's membership in the sphere of Islam in favor of glorification of the realm's ancient, pre-Islamic historical glory. (And the Islamic revolution which ousted and succeeded him in turn sought to reverse much of that and focus instead on Iran's role as a Muslim society instead...)
Thus, if Egypt for instance had both a survival of Coptic among a significant part of the populace and a radical nationalistic movement that viewed itself as modernistic and not bound to traditional views of Islam, and some of those Copts were major leaders of the movement and the Coptic ethnicity (presumably Christians by tradition though perhaps the revolutionary ones would turn their backs on their confession as much as their Muslim-backgrounded comrades ignored Islam) proved active and crucial in imposing the new regime, I could then see the new government actively backing Coptic under a patriotic banner. I doubt very much they'd seek to impose it on Egyptians who were committed to Arabic, which would still be the majority language.
It would be more likely something like this would happen if the struggle for power of the nationalists (who might well be socialists, or even members of the Third, Communist, International) was a hard and bitter one against say a British colonialism that in an alt-timeline decided they'd better stay in power at all costs, rather than as OTL a gradual retreat toward more distant forms of indirect rule that in turn allowed a simple coup to oust Britain's chosen king.