Maybe the tag translation is a desktop feature? Unless it literally came out like 2 days ago, the mobile app still doesn't do that (it's mostly what I use for viewing, and I've got it set to English). Some people do tag in English, but they're the minority. Also, I got an official email recently that was in Japanese; as far as I can tell, it was something about how Booth works. (They do have stuff like password reset emails in English, though.) However, I do agree it's getting better; when I first made my account, the "Read More" button on the site was only in Japanese. Currently, I use Google Translate to get a rough idea of what's going on with author captions, but it's really bad at translating Japanese, and you can't highlight novel text or tags in the app.
As for "terrible art," I don't mean screenshots or 3D graphics (almost never done well, btw, especially with scat), I mean crappy drawings. Some people do manage to have loose, minimalist, or sketch-like styles that are still attractive, but twice as many others just clog the tag with stuff you can't even imagine into being hot. (And then there's the guy in the scat tag who you can tell has good drawing skills and is apparently into hyperscat, but has no idea how to present anything as an erotic focus. Most of us aren't 12; the mere idea of a fetish isn't enough.)