If you are in France with a French iTunes account then you will see what is in the French ibookstore - Apple can only sell in each country what the content providers grant them licenses to sell, so the book publishers may not have made many (or any) of their English language books available to the French store. You can try requesting that they be made available in French store, but ultimately it's the publisher's choice : http://www.apple.com/feedback/itunes.html
Do you have access to other ebook stores/apps e.g. Amazon/Kindle, Nook, Kobo - here in the UK the ibookstore doesn't have as much choice as other stores, and ibooks are also quite often more expensive.
Another way of explaining: If you have a Mac, you can set the language.
If a language set, for example English, the computer behaves as an english computer, but User Data are not changed (that would be a translation): type a doc in greek, it will stay in greek, to change that you would need to translate it.
So if you have books in your Library, they do not change if you change the system language, same for other user data: songs in english will be songs in english also when you set the system language to french. your letters also will not be translated.