Bram -- for Abraham; was Van Helsing a self-insert? -- got a few things wrong about Vlad Dracula, such as calling him a Székely when he wasn't one, and for some reason decided to put him in Transylvania rather than in next-door Wallachia where he reigned, but for the most part he gets the historical stuff right. It's kind of hilarious to think that after what must have been countless hours of painstaking research into an obscure fifteenth-century Romanian voivode, Stoker managed to overlook the fact that the Dutch don't generally speak German, the Dutch speak Dutch. It's like a built-in mnemonic, Bram!
Now, I don't deny that the Dutch can learn other languages. But Van Helsing, resident of Amsterdam, is clearly a native German speaker, prone to mein Gott-ing when he gets emotional. Why not make him Von Helsing, from Berlin, if German was what you wanted? Is it possible -- it's hard to believe, but is it possible that Bram Stoker didn't know that the Dutch speak Dutch? And that no one told him before the book went to press? Or did he really like the effect of having Van Helsing mangle his sentences in a stereotypically German way, but not for some reason want to make him actually German (or Prussian, or Austrian)? Did this have something to do with Otto von Bismarck, Bram?