I bought this on a recent trip to France, as I study French at university and wanted to try reading this in French (I've already read this in English twice).
It's surprisingly quite easy to read - there's only the odd word here and there which I don't understand - of course it helps having read the English version, but you can't remember everything. Some of it is written in the passé simple, but I've not had any problems understanding the verbs.
One of the things I did find odd - not all the characters names are the same as they are in the English. The most strange being that Severus Snape is called Severus Rogue in the French translation - I didn't know this as I haven't read any of the previous books in French.
Otherwise this is quite a faithful translation and it's interesting to see how all the Harry Potter vocab has been translated into French like 'Mangemorts' is Death Eaters. And as this book is written so French children/teens can read it - the language is quite straightforward. So I thoroughly recommend this to all students of French from about 6th form level and above, and anyone who likes reading French books - buy it! - 5 stars!