States start out tied to a single language, nationality, and racial composition, but as populations expand and intermingle, these change: small speaking populations switch to more common languages, racial compositions converge, and communities take up agriculture when surrounded by other farmers.
Conflict and trade decisions are still made at the state level, and states can still expand and break up over time, though these mechanics both affect and are affected by the new variables. Polyglot states tend to stay that way, but if a state with a strong dominant language absorbs neighbors, the conquerors will either convert people to the dominant language or find their own language subsumed into the conquered's (if a resource-rich state is able to conquer a more populous one). When nations break up, the lines of fracture are more likely to fall along national and linguistic lines, which has the potential to have states overextend imperialistically and then balkanize.
I think one missing possibility here is when there are a bunch of adjacent fellow national/linguistic minorities in distinct neighboring states (Kurds would be a real life example): they should be able to break off together into a new state (or be absorbed into a similarly composed neighbor), but right now the decision about whether a state can break apart is based only on its own size.
This screenshot highlights the state of Ridhef, in which after 50 years of history, its founding Rid people still constitute the majority, at 61% of the population. They speak three dialects of their original Ridchiz language (languages branch into dialects when their speaking populations are cut off from one another).
The Ungyech are a sizeable minority of 17% (their original state was probably conquered), and while many speak one of the Ridchiz dialects, others speak Ugea (not sure if that is their original language, as languages are currently named by adding a suffix to the nationality name and "Ugea" seems pretty far from "Ungyech").
Also significant are the Thithe and Wid people, at 10% and 9% of the population respectively. Thithes speak two different Ridchiz dialects as well as five dialects of E'pez (again, not sure where that came from, does not seem etymologically related to "Thithe"). Wids mostly speak three dialects of their original Widmi language, and some speak Ridchiz.
There are three more small minority nationalities, whose populations probably migrated rather than being conquered: the Se speaking their original Sebezh, Echash speaking Ngiegem and Ethe speaking Neqe, though only the Se don't have a majority speaking Ridchiz.
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