Norman lords ruling over Saxon provinces. There is a minimum date on the culture events too, it's early 1100s but don't remember the exact date.
Here's a more detailed response. It roughly works like that yes.
Crusader kings 2\events\culture_conversion_events.txt
Type in ctrl f and search for "# The English Melting Pot".
The most relevent part is:
trigger = {
year = 1100
culture = norman
any_demesne_province = {
kingdom = {
title = k_england
}
culture = saxon
}
}
option = {
name = "EVTOPTA55002"
culture = english
random_demesne_province = {
limit = {
kingdom = {
title = k_england
}
culture = saxon
}
culture = english
}
}
In English that (and the rest of that file) says: When a landed noble is Norman and owns some Saxon land within the Kingdom of England, he will eventually become English in culture. Note that technically this means a Norman king of Sicily owning a single county in England would become English... for basically no apparent reason; but that's uncommon so I wouldn't worry about it.
In:
Crusader kings 2\events\culture_conversion_events.txt
Type in ctrl f and search for "# It's spreading".
In English: Basically makes it so that any Norman or Saxon provinces in the realm of England (de facto) will eventually become English if the King of England is English.
In:
Crusader kings 2\events\culture_conversion_events.txt
Type in ctrl f and search for "#Make my court speak English"
In English: If you are English, not the king of England but are in the (de facto) realm of England and have norman or saxon courtiers then those courtiers will eventually become english in culture.
In:
Crusader kings 2\events\culture_conversion_events.txt
Type in ctrl f and search for "#Make my vassal speak English"
In English: If you are English and are in the (de facto) realm of England then your Saxon or Norman vassals will gradually convert to English. This applies to Norman Vassals in the region of Sicily as well.