https://www.academia.edu/37279899/Sociolinguistics_Oxford_Handbook_of_Roman_Britain_?auto=download&email_work_card=download-paper
of interest to a few on here I suspect. Fascinating short appraisal of the development of language in Roman Britain
Ooh-er. I have just been reading the chapters in Blood of the Provinces (https://www.amazon.co.uk/Blood-Provinces-Ian-Haynes/dp/0198795440/ref=sr_1_1?dchild=1&keywords=blood+of+the+provinces&qid=1611058382&sr=8-1) on the spoken and written laguages of the auxilia (and legions) within the Roman army. The conclusions are quite interesting.
In terms of spoken language, when initially raised, the soldiers probably spoke their mother tongue between themselves, but this rapidly changed when the unit was posted to a new province and started recruiting from the locals. In this case, latin (in the Western Empire) or greek (in the Eastern Empire) became a common language that all could understand, although the grunts tended to use more vulgar versions of these classical languages (nominative words where a different case would be more correct). Some latin/greek was also required to understand senior officers who, unlike the practice of British officers and NCOs in Egyptian, Indian, etc, service, were not required to learn their men's vernacular.
Regarding written language and literacy/numeracy in general, the Roman Army was considered above average, at least partly due to the various administrative roles, such as tax collection, that it was called upon to perform.
thanks Nick, the article mentions the change of form for Latin as well especially in the lower echelons of society. fascinating subject matter all round