i'm not saying english isn't important in the world nowadays, i'm only saying that it is far from being the "lingua franca" ... and a lot of metal bands do release music in their own languages ... but bands aiming for a wider audience in the world, do write in english yes, because if you are going to talk to foreign people, than english probably is your best option that both will speak it. but so many people in so many countries can't speak english, and can't understand those lyrics.
it has more to do with domination first by the english and now by the americans, than that it has to do with english being a lingua franca like latin or french was (and even then less than 1% of the population in europe spoke it!).
even in the scientific world you have to know english, french and german, and preferably even some other languages that are used in your own domain.
but this has nothing left to do with the topic of this thread
... fact is: the book is written in french by a french guy for the french market and this is completely normal! so you can buy the book for the pretty picture on the cover, or you can read it (in french!) or you can wait and hope they'll release it in english someday (or whatever language you want to read it in).