Anne, Llundain doesn't strike me as being too far removed from London, especially in its earlier forms of Londinium, Lunderwic and Lunderburgh.

Not only have the names and spellings of places changed over time, but the pronunciation of both the native and foreign languages may also have changed. Add to that the difficulty most people have in pronouncing foreign words.

It seems entirely possible that, for example, the French "Londres" once represented a fair attempt by someone whose native language was mediaeval (or earlier) French to imitate whatever London was then called in Anglo-Saxon. Once the word gets written down (also bearing in mind that the rules of spelling weren't yet established), it will then be pronounced like that by people who have never heard it spoken by a native speaker. Over time, they diverge until they bear little resemblance to each other in the modern versions of the two languages.

I grew up calling the Chinese city "Peking". Then it was decided that the established method of transcribing Chinese into the Western alphabet was incorrect, and it became "Beijing". These are both attempts to represent the same word. I suspect that the way it's pronounced by a native of the city is different from either version.