sherlock said:How about a mechanism inverse and similar to nationalism.
When nationalism becomes 0 a new "count" begins, with a percentage chance (checked yearly) of culture change.
This way after 40 years of occupation you'd have a 1%/year chance of cultural change, increasing by 1% every 10 more years.
Good one! Very good! Then the chance is present, but not especially big. But if you as France holds f. i Kent from f. i. 1452-1799, they would have a fairly large chance of having become French somewhere along the way. 40 years is also an excellent number because it roughly represents a generation, something that makes a good starting point.