Anglo-Norman language
The English "William" is taken from the Anglo-Norman language and was transmitted to England after the Norman conquest in the 11th century, and soon became the most popular name in England, along with other Norman names such as Robert (the English cognate was Hrēodbeorht, which by regular sound changes would have developed into something along the lines of "Reedbart"), Richard, Roger (the English cognate was Hroðgar), Henry(all of possible Germanic origin and may have been transmitted through the Normans' use of Old French).