OK, after reading this thread, I think I’ve found the solution:
Not intuitve at all, but it seems to work, even when the date validator still recognizes the old dates as text and now I have to change all of them manually from “Estimated” dates (recognized as text) to “Estimated” dates (recognized as dates) by clicking on each event and just pressing OK. 
I wish there was an easier and user friendly option to change language within he program.