Due to phonetic translation from English to Japanese and back, the character known as Michael in the books became "Markl" in the film. To make matters worse, whenever his name is spoken, it sounds more like "Marco" than anything else...
The Swedish and Norwegian dub doesn't only have the same mistranslation, they also call Howl "Hauru", for the same reasons. Slightly justified in that they were trying to lip sync the movie, but it still comes off a bit awkward.
Howl's called "Hauro" in the German dub and the mistranslation of Markl also took place.
This is justified, as in Japanese (the original language of the film), Howl is written (and pronounced) "Hauru" (ハウル). Neither "Hauru" nor "Howl" have a meaning of their own in German, anyway, and the original book is all but unknown there. The German title just translates to "The Strolling Castle".
The same thing happens with the name of Sophie's (step)mother. In the book her name was "Fanny". While she is never called by name in the movie, her name is written as "Honey" in the art book. The romaji for both words is different (Fanny = ファニー, Honey = ハニー), but the pronunciation is nearly identical ("Fanī" and "Hanī", respectively).
What Could Have Been: Director Mamoru Hosoda was originally brought on to helm the project, but was taken off during the early stages of production.