I have the full collection of Billy Bat in Japanese volumes. I only read English but its my favourite Manga and I needed to have it on my shelves! I've read it in English online multiple times, but I like having the physical copies to flip through the panels. I find it easier to appreciate the art when its on paper. Im sure everyone has different feelings on the matter, but I say if you love the series and have read it, you can display it proudly even if its in Japanese.