Depends on the language. In Italy this was a huge issue, as Latin persisted as a "prestige language" such that there is a dearth of written evidence documenting the transition from early medieval vulgate to the various Italian dialects.

In the early 14th century, a clique of very influential poets and writers from Tuscany emerged who began writing in the local vulgate. Because of the enormous popularity these poets (Dante, Boccaccio, Petrarch, the list goes on...) when two upper-class Italians from two different states met, if they weren't speaking in Latin they would most probably end up speaking to each other using Tuscan literary vocabulary. However, accents and pronunciation was far from this standardized "Literary Tuscan" (which retained multiple features that spoken Spoken Tuscan shed). With no "Italian State" there was no "Italian Capital" or dominant city with a prestige dialect to imitate. Italian writing would also "Tuscanify" as the early Italian literary canon was mostly Tuscan, even if pronunciation and elements of vocabulary varied widely across Italy and still does to this day. Linguist Tullio De Mauro estimated that some 2.5% of the Italian Population was able to communicate in Standard Italian (i.e. Literary Tuscan) by 1861 (other, more generous estimates place the number at 10%). To this day, there are notable divergences between what is acceptable in written and standard Italian.