This is like a plot point in the old sci-fi movie Colossus: the Forbin Project.[0]

In the movie, America and the Soviet Union have both developed an AI. The two AIs are linked, and they rapidly shift from speaking human languages, to speaking in sequences of numbers that the onlooking humans can't understand.

Spoiler alert: this all goes horribly wrong for humanity.

[0] https://en.wikipedia.org/wiki/Colossus%3A_The_Forbin_Project