In linguistics we have a concept known as 'language relativism'. It's the idea that since languages change with time, and that they can share features with each other, the borders between them can be quite fuzzy, with two people who speak the same language merely having a very fuzzy border between their personal languages, with dialects merely making these borders more noticeable.
This concept sees languages like nations, defined more by agreement and consensus then by any real set of defining characteristics. It's why dialects aren't languages, since non-dialectals can still understand them, and why – even if you don't speak Japanese – you could still understand words like 'konnichiwa' or 'arigatou' in a way that feels like English.
If you want to see linguistic relativism at work: watch this video on Scots.
Any-who, let us focus on ancient Europe and the various degrees of ængles the peasants probably spoke.
In learning linguistics, we're taught how people in medieval Europe often lived in villages, and would rarely stray far from their community. Thus, people developed accents and dialects, ones much heaver then the those today (imagine if every dialect was a deep Scottish one).
Due to the relative nature of language, speakers could often speak to neighbouring communities, with nearby communities having more communication and thus more of an effect on how people spoke. This resulted in people having a linguistic circle of influence, a certain range of neighbouring tongues they could understand, with these neighbouring dialects getting heavier and heavier the further they went from there community, eventually reaching a point at which they simply couldn't understand them anymore, with these dialects having become languages from their point of view.
How would this have played into life back then? If communal languages made it so that a kingdom could contain countless communal languages, each linked in terms of who could understand who.
In my classes we learned this stuff as it's a really good example of relativism at work, but we never really explored the history it brought. Thus, I'm simply very curious as to what this would have done to medieval life.
Did armies have to deal with this variability in understanding? Did tax collectors and town criers need to be communally picked? How much is written about people back then using dialects to identify communal origin?