Related question: why do we translate some monarchs' names, but not others, sometimes even from the same country? For example, tsars/emperors of Russia named Ivan are not usually (or ever, as far as I've seen) translated to John. But, those named Nikolai are often translated to Nicholas.

To the other comments, I just want to add--don't overlook the power of scholarly convention. It's not just monarchs' names that are translated. It's whoever was talked about in scholarship and popular culture well into the 20th century. The move to leave names in the original--I read a couple books for my PhD exams that had Spanish kings as Jaume instead of James--is relatively recent.

A lot of medieval books have a note in the front along the lines of, "I've used a translated name where there is a traditional one, for ease of recognition; other names are the originals with standardized spelling."

The most egregious example, in fact, isn't a king--it's Joan of Arc. Jehanne d'Arc in French...with "d'Arc" not being "de Arc/of Arc," but rather the surname "Darc" which could be either Darc or d'Arc.

Except...d'Arc is her father's last name, and according to her testimony at her trial, girls in her region took their last names from their mother.

But if you wrote about Jehanne Rommee or Jehanne de Vouthon, no one would know who you were talking about. So Joan of Arc it is.

I address a similar question here: https://www.reddit.com/r/AskHistorians/comments/9e179j/whats_the_deal_with_historic_names_being/e5lxmh5?utm_source=reddit-android