Why was he called Charlemagne?

His given name was Charles in French, Karl in German, or Carolus in Latin, with some variation in spelling. He was given a title equivalent to "the Great." So in German he is called Karl der Grosse, and in Latin he is Carolus Magnus. Charlemagne is the French name, and it is also used by the English.

He is called Great because of the various things he did. He was crowned king of a powerful kingdom, but he increased the size enormously, making it the largest country of Europe after the Roman Empire. He was crowned Emperor of what we now call the Carolingian Empire, in 800, by Pope Leo III, and this empire was ancestral to both the Kingdom of France and the Holy Roman Empire. He converted large numbers of people to Christianity. He pushed the boundaries of Christian countries farther to the East, against pagans, and the South, against Muslims. He protected the popes and the Church. He founded schools and encouraged education, making a policy that all free people of his lands have access to education, and seeing to it that both men and women around him learned to read and write. He spoke several languages, including Latin, Arabic, and at least some others. His time was called the Carolingian Renaissance, though it was named for his grandfather, Charles Martel, who was regarded as the ancestor of the Carolingian Dynasty.