the reason why the slavs and the celts have long held that a nation must have its own distinctive language has to do with maintaining the decentralisation of government and law.

it's one of the features you see in all large, tyranical, imperial systems, the spread of a single form of language.

the romans did it, the bulgarians did it, the russians did it, the serbs did it, the germans did it (very recently), and the longest lasting and most lethal form of it today is the english language.

i had recent disagreements with some people about their view about the eventual universality of english, and pondering the nature of law and its centralisation and weaponisation reminded me that part of the way to establish boundaries between groups of settlements is to make distinct languages.

i observed this very clearly in the border between Bulgaria and Serbia. They literally and clearly defined their differences from each other in the language, making it difficult for a person raised speaking one language, to speak another. A simple example is the way that bulgarians and serbs differently say "because":

Bulgarian: zashtoto - literally "the why"

Serbian: Zato Shto - literally "for this why".

the differences in their conventions for the definite article are at play here too, the bulgarians often use the definite article, which they construct similar to nordic languages, as a suffix, as indicating generality instead of singularity.

having learned the bulgarian first, learning the serbian was always a tongue twister.

there is probably hundreds of examples in the similarities and differences between these two particular languages. Phonetically was one special point too - the L in bulgarian is more like the russian, very round and distinct, whereas in the very same, otherwise, phonetically, word, the serbians drop the L altogether, even though it is found in other related words. Like Beograd, which is pronounced almost like the bulgarian way to write it, Belgrad, which means "white city". Bulgarians say "Delo" and the serbs say "Deo", another example.

the real back story about why regions develop strongly distinct ways of speaking is all about making their legal systems distinct from each other.

this is even enshrined in the story of the Tower of Babel, which isn't about events even that far back, in fact, literally not long after that time (around 500BC) the romans started spreading latin around and building their empire.

setting up schools and teaching languages is part of the process of colonisation and development of imperium.

common people fight against this, and many people mistake this prejudice for being misinformed or low intelligence.

but in actual fact, it is wise to maintain a distinct way of speaking in small regions, if you don't want your local government to become an arm of an imperium.