I'm reading "The Crusades" by Thomas Asbridge.
He quotes a Crusader song apparently from the 2nd Crusade (circa 1140 CE) and apparently written in old French. Yet it rhymes in English, nearly all the way through.
Even if it was written in English, I've seen some examples of middle English and I would be surprised that it still rhymed!
I went looking for the original so I could translate it, but found a side-by-side translation to save me the trouble. If you compare the two translations, you can get some taste of the variance a translator can introduce into a text. The one in your book tries to translate it into a singable version, using a similar (but not identical) meter and rhyme scheme; the one in my link above tries to capture the meaning of the text more accurately, but discarding its form. And then here's another that is more word-to-word comparable but suffers some in contextual information and also couldn't be sung.
None of these is more right, this is just a decision the translator has to make. There's really no such thing as literal translation. (Well, maybe there is, especially if you involve computers, but it'll fall somewhere on the scale from gibberish to awkward.) This is also the reason that historians are expected to work in the original language of their sources whenever possible: the translator invariably introduces certain decisions into the translated text. If you only read this song in English, you'll get a very different feel for it depending on which translation you chose.
Consider the first line:
These all have very different interpretations on what is promised to those who fight the Turks and Almoravids, and even who the audience is.
Ttranslating poetic work in particular is a tricky thing. It's why Beowulf and The Iliad and other works keep getting translated, and why many people have very specific translations they will recommend. Someone always has a different take, and frankly it's a poetic exercise itself. Take a look at these four translations of Beowulf (in the sidebar; I'd recommend reading them first, and then the commentary on the rest of the page if you're interested). All well-known, all very different while containing the same material.
Hi, hopefully someone will be able to comment on this book or its author, Thomas Asbridge, specifically, but while you're waiting, you may find a couple of other recent posts on translations of interest: