Is the Ultimax art book worth getting?
Depends on what one values, I guess. Language isn't a barrier because there is practically no text in there at all, only illustrations. There are more pages than the first art book at 238 pages, but it recycles a lot from the first art book. It's padded out with a lot of high resolution images we're already familiar with from the game itself, and it doesn't have a lot of art unique to it.
In terms of concept art, Sho, Ken, maybe Akihiko, Junpei, Asterius, Ariadne, evolved Himiko and
are the only ones with new illustrations in the entire art book which give us a peak behind the design process for the game. The only thing else that's unique to the art book are 26 pages of storyboards for some of the animated scenes. The rest is just padded out with stuff we've seen before; there aren't even anymore storyboards for the in-game sprites like there was in the first P4A art book, for the old characters or for the new. It feels like they kept stuff closer to their chest for some reason, with this one.
If one would end up with the art book eventually, it's not like waiting for a localization is necessary since, like I said, there is literally no continuous text in the whole thing except for a brief message on the final page which probably says something that sums up to "thanks for your support." Outside of the recycled character/Persona concept art from the first art book of their final designs in a T-pose, there aren't any comments from the developers at all associated with any of the illustrations, aside from typical stuff for the storyboards. There isn't an interview section, either.
Basically, it's a standard art book with high res. illustrations one would expect, but there's not much of note in here other than the few pages unique to it like I mentioned above. I find it less interesting than the first P4A art book because of that—it felt like they were more willing to elaborate on what occurred behind the scenes with that one, and it even had unused stage concept art—but it still has pretty images. Again, not worth waiting for a localization if one really wants it and could import it for the same price because there's essentially no text in here.