In the episode 1.04, we get a very import information about the moon base creation.
I can't access the show right now to give the precise script, but basically Crystal (the blond princess) says they use Lockjaw (The big teleporting dog) abilities to go to the moon.
That means they went there during their life (or at least during the life of Lockjaw)
In this case, it makes sense they speaks an existing human language.
You can still argue it would make more sense that they speak Chinese or Spanish, because it would be way easier to hide a city in the heart of China/Amazonia than in the US.
But it now revert to the classic "Everyone speak the native language of the watcher" question.
Edit after 1.07
First of all, forget about everything above, i misunderstood the line, they were talking about the trip from moon to earth (relating to the show), not the migration from earth to moon.
But, and more close to the question.
They clearly don't write in English on Attilan.
Yes, the question was about speaking, but speaking and writing tends to be related.
The flag could still be a 'high speech' thing, used only in specific occasion. But the Karnak scene implies this alphabet (and the matching language) is his thought language.