Almost three centuries after the conquest, Geoffrey Chaucer, taking over as clerk of the King's works, was handed an inventory of stuff left lying around at the Tower of London, from which the following extract is now quoted:
"I ramme cum toto apparatu excepta i drawying corda que frangitur et devastatur, i fryingpanne, i lathe pro officio carpentarii."
(...a battering-ram with a winding-cord too damaged to be usable, a frying-pan, a lathe for a Latinised carpenter.)
This document was compiled by a fourteenth century white-collar worker who (like Chaucer) juggled three languages in his head on a daily basis. First there was Old English, the speech of the masses, by then grammatically-condensed and well on the way to becoming modern English. Then there was the elite dialect, Anglo-Norman French, and lastly Latin, the preferred communications system of the Church, public administration and cultural transmission in general.
Textual intercourse between the English and their new rulers had kicked off almost as soon as the old order had been crushed on the field of Hastings, such that many basic English words quickly picked up romance accretions. (Water...aquatic, aqueous etc.)
But the text above should not be read as an example of some smart-arse peppering his script with French or Latin insertions — as certain modern hacks might do, just in order to show off what a pretentious tosser on est enclin à être. There was a deeper, more reflexive linguistic interpenetration at work here; a process known in the field of poetics as macaronics.
Educated scribblers of Chaucer's day would tend to pick whichever word, phrase or even grammatical construction seemed more familiar or apposite without fear of fostering incomprehensibility, because most of their readers would be similarly blasé about the boundaries between their vocabularies.
Modern scholars debate the extent to which this fusion was reflected in the spoken language of the time; another example of one of those issues around which academic contention loves to coalesce, because there's ultimately no sure way for anyone to know if their strongly-held opinions are right or wrong.
Fast forward another three centuries and we encounter another fascinating example of commingling tongues in the diary of Samuel Pepys. Like many modern bloggers, for whom he was the great precursor, Sam was never quite sure if he was addressing an audience of one, or whether his words might indeed become of interest to a wider group of readers, possibly including posterity. Yet regardless of the tone employed, Pepys encoded all his daily musings in shorthand...just in case his wife Elizabeth should locate the diary and proceed to peruse its franker admissions. In modern geek parlance, the shorthand was his entry-level firewall. Access-levels further down were assigned via linguistics.
As an OP Pepys would have acquired his Latin and Ancient Greek at school and would surely have picked up a smattering of French through contact with Elizabeth's family, refugee Huguenots. Yet surprisingly, when slipping into his most intimate mode of discourse, and when conscious of having done something really naughty, Pepys writes in Spanish.