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A total lack of Roman literature on popular topics is also present: apart from Apicius' cookbook we have no mention of comedians writing in Latin for example.
Since comedians usually write their plays or jokes for the masses that would be a sure way to prove that the masses could understand Latin; sadly no such originals exist.
Now the arguments above do not even take into account the fact that originals of most Latin works are lost and only (((copies by monks))) exist.
For example there are no surviving originals of Livy or Marc Aurelian as mentioned previously and this is also the case for Plato, whose works got ""discovered"" by Gemisto Pleton in the 15th century.
Regardless of all of this there would still be a rather good way to determine the spoken language in the suburbs/poor neighbourhoods: the use of graffiti/abbreviations/carving on the walls there.
Let us focus on the use of graffiti by the Romans or rather the near-entire lack of it: take a good look at the pic in the OP post.
This would be a so-called political graffiti written on the walls of Pompeii and other graffiti is reported to have been found near bars with more explicit texts.
The first and foremost oddity here is the lack of other scribblings over it: as we know graffiti tends to consist of multiple layers because regular punks write graffiti *on top* of another man's graffiti.
Since this is a political graffiti urging folks to vote for some candidate we would expect to see at least some scribblings or maybe penis drawings from his political enemies at the very least.
But no, since someone posted that message at that wall no single Roman scribbled over it in Latin.
Now it is widely known that carbon dating does not work and we all know about the mud flood hypothesis: it becomes evident that this graffiti was added onto the wall later by (((someone))).