When visiting a book store in any [EU] country, have you ever bought a book written in their own language instead of English? (If you are a native speaker or an advanced learner of their language.)

I knew a friend from overseas who visited bookstores in Japan other than Kinokuniya (as most of their books are in Japanese, with a limited selection in English depending on the bookstore) but of course Japanese is the main language you will find the majority of literature in their country. (It's the same in any country you go to, since you'll find more books in their language.) You can buy a novel in any language (i.e. French), as long as you're able to understand the content written being past A1-B1.

There is a "difference" between a reader who does not speak or understand the [language] at all (other than English) when they look for books while in a [EU nation] only seeking for English books versus a reader who is either a native speaker or an proficient learner on that [language], since they are able to read the blurb and understand the book's title without looking up any of the words on a dictionary. In terms of novels (not comics) how proficient do you have to be in that [language] to fully understand everything written inside a novel?

Regarding Japanese & Taiwanese Mandarin books, they are very different in terms of how they are printed (from right to left with vertical text) unlike in most European languages including English they are from left to right with horizontal text, so it's a "different" reading experience in that sense, not only because of the language, but how books are published. Japanese books (even the dust jacket has an additional flap) are smaller and compact in size (I'm talking about you, Dutch books!) than European & English books where they are larger in both hardcover and paperback.

For instance, it would be like if a individual from the UK went to a bookstore in the Czech Republic (of course all of the books are going to be in Czech first, with a limited selection of English books again depending on the store and location) unless they have learned Czech before (being proficient on understanding every word, at B2-C1 level) then they won't really understand everything written inside the book since it is intended to be for native speakers, who are reading these novels.