In Germany he is known as Karl der Große, while the rest of the world knows him as Charlemagne or Charles the Great. I read that his actual birth name might have been Karolus, so I was wondering how and when 'Charlemagne' came to be the dominating name for him to the rest of the world.
Essentially these are all the same name, just a variation/translation. Karl comes from Germanic karal meaning man or husband. The Latinised version of Karl being Carolus (Carolus Magnus with Magnus meaning the great), French Charles. The reason that English uses Charlemagne and not Charles the Great (which is literally the same, just different language) is the influence of French on English following the Norman invasion of England in 1066.
Now I'm not so sure if Charlemagne really is the dominating name in the world. Some examples for other languages are:
Danish/Swedish: Karl den Store
Croatian: Karlo Veliki
Italian: Karlo Magno
Dutch: Karel de Grote
Russian: Карл Великий
Polish: Karol Wielki
Portuguese: Carlos Magno
German: Karl der Große
English/French: Charlemagne
All of these meaning the exact same, Karl/Charles (translation/variation of the name for the language in question) and great/the great.