In reply to Ffat Boi:
English, of course. The book I am reading is called "The Inheritance of Rome" by Chris Wickham, part of a series called "Penguin Histor of Europe"
On page 176 he e.g. writes in the context of the Christian takeover of pagan holy sites, rituals, and festive dates like Easter in the 7th and 8th century, that in contrast to Willibrord, who worked amongst true pagans in Frisia,
"...Boniface worked mostly in central Germany, fully part of the Frankish world, ... , and the 'pagan' practises he describes there were more likely local Christian customs, like those at Noyon. .... simply preaching against such customs was unlikely to get rigorists very far, precisely because they were seen as Christian already."
CB