On Firefox (locale is English): I speak Whatever, I want read Whatever else. Choose any book. Voice is English. The only exception is if I choose I want to read Spanish, but this is because I have installed a particular Spanish voice in my system, and that's the one that's used.
On Vivaldi (browser's locale is Spanish): Anything is read with a Spanish voice, unless I choose I want to read English, then an English voice is used.
Also, in the german text the chapter markers ("Erstes Kapitel", "Zweites Kapitel") have been pulled into the paragraph following them, but they're missing completely in the english text.
On Vivaldi (browser's locale is Spanish): Anything is read with a Spanish voice, unless I choose I want to read English, then an English voice is used.