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When you are talking about serfs bound to their village, that would be in the middle ages. Armies in continental Europe were feudal. So were tax collectors. Documents were often written in Latin.
National armies and tax administrations belong into the early modern period. At the time, the upper classes would often speak French, regardless of what their subjects spoke.
Upvote:5
Against the centrifugal effects of local dialect formation, there are the moderating effects of trade networks, and the centripetal effects of central authorities. While central authorities tend to be weak before the modern period, especially in medieval Europe, there was the significant role of the Catholic Church and its network of priests who relied on Latin, and the relation of intellectuals to the Church.
In late medieval and early modern Europe, the rise of nation-states explicitly involved the development of national languages, which were intended to supplant both local dialects used by most people, and Latin used by intellectuals. A major reason for this was to overcome the sort of practical difficulties of organizing state institutions, such as standing armies and rationalized taxation and trade systems -- while asserting independence of the Church.
The idea of a national language was most famously discussed among writers and intellectuals in Florence, Italy, in the Italian Renaissance. This Wikipedia article on the Italian language gives a good overview. This influenced thinkers elsewhere in Europe. As the article notes, Dante Aligheri's Divine Comedy had a significant influence on modern Italian. A great deal of Divine Comedy is commentary, direct and indirect, on political struggles between factions in Florence.