I was looking up some information about the Knights of the Round Table, and I'm surprised that so many characters from the Arthurian legends have French names. Breunor le Noir, Hector de Maris, and Aglovale de Galis, to name a few.
While the original Arthurian stories have an origin in Welsh stories such as those surviving in the Mabinogion they were popularised and promulgated in Anglo-Norman and Breton twelfth-century societies before being picked up by southern French troubadours. While the image created remained geo-narratively centred on the south-western coast of England, Brittany, and modern Wales, these authors were reshaping Arthur in their own image of what an ideal society would be.
The Arthurian romances (typically after Geoffrey of Monmouth who wrote in Latin) were also a major strand of vernacular literature, so it is unsurprising that the Old French/Anglo-Norman/Occitan names have been retained in Anglophone societies.
Arthur is a legendary English king, but the medieval Arthurian literary tradition is transnational and multilingual. Thanks to the Norman conquest, prestige of French (Anglo-Norman) as the courtly language even in England, and English landholdings on the continent, England and France in particular develop closely intertwined literary cultures. The 'central storyline' of Arthur's story, along with the most important stories of Round Table knights, evolve through Latin, French, and English. (And other languages cultivate their own variations. There might be some influence in the main tradition from Welsh, too.) In particular, the French romance tradition is really where the knights and their adventures are developed, as opposed to the tragedy of Arthur. The most obvious evidence of the importance of the French tradition even in England and English is that Thomas Malory calls his very English text Le Morte d'Artur.
Breunor le Noir comes from the Prose Tristan or Tristan en prose, a 13C French version of the Tristan and Isolde tale. Ector de Maris or Hector of the Marsh comes out of what we call the Vulgate Cycle or Lancelot-Grail cycle, a series of 13C French texts. Aglovale also appears in the Vulgate cycle, but he plays a larger role in what I swear is called the [French] Post-Vulgate cycle, because apparently we gave up on creative names for things, and then in Malory's Morte.