The Dutch girl that spoke Dutch? She was speaking real Dutch, the production team took the time to find a child who spoke the real language. Thus ensuring the scene's believability.
The same with the little deaf girl in the 1994 version. They changed it from a Dutch girl because the original had her as a war orphan and that would make far less sense in 1994.
Along that line, Santa Claus speaking Dutch may be a Mythology Gag, considering the name Santa Claus comes from the Dutch language word for Saint Nikolas, and St Nick himself is the Patron Saint of Amsterdam
In the 1994 remake, either Santa did a Virgin Mary routine on Dorey, or he knew exactly when she was ovulating to orchestrate a liaison so she could conceive a child. Neither option is particularly appealing.
Assuming she was pregnant at that time. It's possible he counted Dorey getting married on Christmas as a set-up for Susan's wish for a sibling.
It was heavily implied. That's enough to be horrifying.
In all versions, the thought that they nearly committed the REAL Santa Claus to a mental institution is horrifying.