Fifty years ago, my school history textbook for the French Revolution described a king called "Lewis XIV". Even then, I thought this strange. Why translate "Louis XIV" into "Lewis XIV"? You lost information ( you didn't know what the French called their king) and it wasn't necessary to do so: "Louis" is a perfectly good name in English. Fifty years on, I can only think that the author of this old textbook was a Scotsman - in Scotland, "Lewis" is almost certainly more common than "Louis".
But the author was only doing something we do all the time. In English, the Maid of Orleans is always called "Joan of Arc" and never "Jean d'Arc" or "Jean of Arc". In inconsistent contrast, 'l'Arc de Triomphe is never called " the Triumph Arch" or "the Triumphal Arch" but usually "the Arc de Triomphe". And the Champs-Élysées is never - but never - "the Elysian Fields".
If I was writing a style book (as I am now), I would start with one rule: Avoid translations which lose information unnecessarily. Many English versions of capital city names fail to meet this simple requirement. Consider three examples:
"Roma" is rendered "Rome" in English, so we lose a bit of information about what the Romans do in Rome, namely, how they pronounce the name of their capital. There appears to be no reason why "Roma" should not be rendered "Roma" in English.
"Wien" becomes "Vienna", a nicer name it's true but partly misleading. It's maybe helpful that the "W" becomes "V" to reflect differences in how these letters are pronounced in German and English, though it's very rarely done - "Weimar Republik" is never rendered as "Veimar Republic" . In fact, German place names are usually left unaltered: think Wiesbaden, Wuppertal, Worms ...
Back to Vienna. Why isn't "Wien" either retained or rendered as "Vien"? What is the " - na" doing on the end?
"København" becomes wonderful, wonderful "Copenhagen" This is massively defective. First, there is no need to change the "K" to a "C" to indicate the pronunciaton - "K" works in English. Second, it's true that when accents (diacritical marks) are missing from our keyboards we need a substitution rule, but in this case the correct substitution for "ø" [ for which I have to type Alt + 0248) is "oe" which gets closer to the Danish pronunciation than the completely incorrect "o" which in English leads to the pronunciation "Cohpenhagen". Third, when "havn" becomes "hagen" we lose the information that this city is a port, a harbour, a haven. But there are many English harbour names which include the ending "-haven" (Peacehaven, Newhaven, Whitehaven ...) and only the very dim-witted would fail to pick up the link between "havn" and "haven". In addition (and fourth), "-hagen" removes information which might help us to the correct pronunciation: it takes a way a soft "a" and gives us a hard "a". Fifth, we have this slippage from "b" to "p" which is unnecesary even though there is a complicated issue about "b" and "p" pronunciation (just as there is for "r" and "l" in Chinese).
Result? In English, and in the absence of Alt + 0248, the Danish capital should be referred to as "Koebenhavn"
I could go on at great length. Even more fun can be had when you get into transliteration. The Cyrillic of the Russian capital transliterates as MOSKVA and if you pronounce that in English you get close to the way Muscovites talk about their city. But "Moscow"? Forget it.