>I believe Arabic is the language with the largest variety in sounds.
You believe wrongly. There are languages with far bigger phonological inventories. Practically every language spoken in the Caucasus region would fit this criteria.
>English in the 10th century was basically the same language as German (Althochdeutsch or Old High German, to be precise). In 1066, the Normans conquered England, bringing French which became the official and court language of England for several centuries.
Old English and Old High German being "basically the same language" is a very ignorant thing to say and categorically wrong. Old High German might have been much more close to Old English than modern standard German is to English but they weren't mutually intelligible and they were clearly different languages even at that time. Major differences between the two had arisen many centuries earlier. Take for example the High German consonant shift. Now, would you please explain why you would say that both were "basically the same language"?
You believe wrongly. There are languages with far bigger phonological inventories. Practically every language spoken in the Caucasus region would fit this criteria.