Would not know; never met a medieval peasant in a Fantasy setting. Perhaps they use proper grammar and have the self discipline to hold their tongues when speaking to those with weaponry....
They swore alright, in the sense that they seem to have had far less hang-ups when it comes to non-religious swearing, using sexual and fecal terms quite a lot in fact. It’s only much later that these terms began to be considered ‘obscene’. One of the reason this type of swearing still has some impact is possibly a result of a certain ‘cleaning up of everyday’ language in the modern era. Medieval Europeans (this also applies to other eras and cultures) may have been less shocked by much of our modern swearing than we ourselves.
Religiously-based swearing is another matter; putting God, Christ etc. in your swearing did occur, maybe even fairly frequently (we know it happened, but not how often) but the Church and local authorities could come down hard on the people who did this. Punishment could range from some time in the stocks and fines to branding and other fun forms of mutilation.
If CDPR really wanted to do a ‘realistic’ quasi-medieval RPG, non-religious swearing certainly would have its place, but defiling the name of major deity X…might lead to the local court ordering your tongue cut out or split in two.
If any CDPR dev is reading this: Feel free to use this as an idea for a quest in future DLC… 