Volgens mij is er maar één historische bron die iets zegt over dit onderwerp, en dat in de biografie van Einhard, geschreven rond 820.
Er is geen andere documentatie uit die periode (dark ages) die iets over dit prive leven verteld.
De biografie door Einhard, van Karel de Grote in het engels vertaald: https://sourcebooks....sis/einhard.asp
Charles had the gift of ready and fluent speech, and could express whatever he had to say with the utmost clearness. He was not satisfied with command of his native language merely, but gave attention to the study of foreign ones, and in particular was such a master of Latin that he could speak it as well as his native tongue; but he could understand Greek better than he could speak it. He was so eloquent, indeed, that he might have passed for a teacher of eloquence. He most zealously cultivated the liberal arts, held those who taught them in great esteem, and conferred great honors upon them. He took lessons in grammar of the deacon Peter of Pisa, at that time an aged man. Another deacon, Albin of Britain, surnamed Alcuin, a man of Saxon extraction, who was the greatest scholar of the day, was his teacher in other branches of learning. The King spent much time and labour with him studying rhetoric, dialectics, and especially astronomy; he learned to reckon, and used to investigate the motions of the heavenly bodies most curiously, with an intelligent scrutiny. He also tried to write, and used to keep tablets and blanks in bed under his pillow, that at leisure hours he might accustom his hand to form the letters; however, as he did not begin his efforts in due season, but late in life, they met with ill success.