Borders of differing nationalities always had friction and intermingling. As humans, it's unavoidable.
Nations with large trade cities had all manner of ideas foreign to them happening around them every day.
Conquering nations spread fear and a different way of thinking/doing in every place they went. Considering the vast empires and travel of the armies of the day, thinking ordinary people couldn't walk that far seems kindof odd.
I'd agree it may be more localized then vs now... And I'd say that heavy trading nations would exhibit the trend alot more than a holed up theocracy... But I wouldn't say that it didn't happen.
Perhaps the historically accurate folk around here could provide a few examples and enlighten us, the poor demographic informationally challenged...