You are presumably referring to the documents compiled after a series of
church councils attended by all the bishops of Gaul, at Mainz, Reims, Tours,
Chalon and Arles (Royal Frankish Annals SA 813). We do know however that
Charlemagne was concerned to keep up the level of Classical education in his
realms (the so-called Carolingian Renaissance).
If you want to see what the vernacular language (lingua romana) looked like
in this period, you need to find a copy of the original text of the
so-called Strasbourg oaths (14 Feb 842) as presented by the historian
Nithard in his otherwise pretty boring "Histories". These texts are renowned
as being some of the earliest written examples of the French and German
languages (a fact not without significance bearing in mind the current
status of the town) and as such there is quite some literature on them. The
text itself can be found at:
I think though that over the intervening years between the 800s and the
Battle of Hastings there might have been some changes in vernacular French.
A quick Google search reveals that there are a number of sites devoted to
the early development of the French language which you might find helpful.
See also for example:
Hope this helps a little,