As far as Bannerlord goes, these guys all speak the same language
IIRC different languages such as Palaic and Dryatic (?) etc., are mentioned. Language barrier therefore probably does exist but is abstracted for the purposes of gameplay when communicating with army leaders, like it is in almost any game set in the real world.
Just look at place names and high-ranking lord titles and it's pretty obvious different languages are used.
have the same religion (none)
"The Sacred Heavens" are alluded to frequently in the Empire and Aserai dialogue, while Khuzaits have some mention of vaguely sounding Mongolian style shamanist religion, the Sturgians have sacred oak groves in their city description, and Battania has its own mythology.
and share a similar culture.
Under what metric? Art, architecture, clothing and armour, music, place naming conventions, weapons used, food eaten, economic focus, all different. The culture descriptions and backstory go out of their way to make them sound different too.
The cultures are evidently quite different which is why we get that massive punishing -3 penalty to loyalty for having a settlement governor of the incorrect culture.
Systems of government are different too - each culture has different default political policies.
And although the game's systems fail to bear this out, rulership systems are different in-story too. Vlandia and S. Empire are hereditary monarchies, N. Empire is like the senatorial Principate/late Republic, W. Empire is like a populist Dominate, Khuzaits/Battania/Aserai are loose confederacies where central authority is barely tolerated or exercised through force, trade or high kingship, and Sturgia is a principality.
Crusader armies (at least after the First Crusade) had Turcopoles.
Yes, but turcopoles were often recruited from native Christians, or even from Europeans. So in that sense they shared major cultural traits with their recruiters. "Turcopole" really just meant "armed
in the manner of a Turk". The Crusader states were largely maintained by mercenaries, the Templars and Hospitallers, and a constant influx of Europeans immigrating from the west. Not from local recruitment of people from a totally different culture, for the most part - though it certainly occurred to an extent.