Yes Eric, it is written in English. Remember he was raised in the US.
I just thought of another very good reason this book might have been written. Not only was Pachmann waiting in the wings claiming to be the real legitimate heir as the Habsburg head of house and whatever fortune there was but there was also another son born after Rudolf died I think and detailed in Listowel's book.
She reported how a famous American Civil War General, James Longstreet, represented the US at Rudolf's and Stephanie's wedding and remained on friendly terms with Rudolf and was then hired by the emperor to track down Rudolf's son after the Mayerling tragedy. General Longstreet was joined in his efforts by another civil war general -- Cassius Marcellus Clay -- who was appointed by Abraham Lincoln to the Russian court where he remained for 8 years and became close friends of the Czar and through that connection became involved with Rudolf and his American son. Much of this information came from the State Historical Society of Missouri. This 3rd son was the child of an illegitimate daugher of the Russian Czar.
So here we have the supposed first son, Deventer, born in 1877 from a legal Catholic marriage between his mother of minor nobility and Crown Prince Rudolf. That certainly trumps any claims made by anyone else, and best of all, Deventer is overflowing with praise for the late Emperor Karl and his entire family.