I agree with the Empire suggestion, but not the singularity.
It's the people's name. It would the difference between, "I'm choosing the Turks" and "I'm choosing the Turkish". Like in AoE.
We choose a culture here, not a people. The text says: 'Choose your character's culture.'
We don't choose the Turks but the Turkish culture.
And it's good this way, because we could choose playing a Vlandian hero who later serves the Battanians or else.