Question: Daniel Handler, aka Lemony Snicket, is a born and raised American, he writes books in English. Now why, in this movie, does the letter the children receive in the end contain the text "Groeten Uit Antwerpen" which is Dutch/Belgian? Maybe the answer is obvious if you've read the book, but I haven't.
Answer: The children receive a letter from their parents, who are on vacation in Europe (in this case Antwerp, Belgium - the phrase means "greetings from Antwerp"). They thought it was lost or never sent. It has nothing to do with the author.
Zaphod Beeblebrox