Japanese characters are actually written left to right, but sometimes the page order is right to left. Writing that you might find on a website, e-mails, and scientific writing is typically actually written left to right. While these kinds of texts may have pages that are ordered from right to left, the text on the pages is typically written from left to right. It is typically only when text is written vertically (yokogaki) that it is written in columns going from right to left, and in that case, the characters are read top to bottom.

Sources: [1] https://www.lingocommand.com/japanese/writing-systems-explai... [2] https://en.wikipedia.org/wiki/Horizontal_and_vertical_writin... [3] I studied Japanese in college lol

When written horizontally it is now left to right but earlier you would see horizontal right to left. But vertical was preferred especially in the past.

You can see horizontal train stop signs written right to left in “In This Corner of the World” anime. Today all signage seems to be left to right.

[edit] The history section in Wikipedia explains that this was a postwar script reform. https://en.wikipedia.org/wiki/Japanese_writing_system

In the time this art was made, top to bottom, right to left was the standard. It's pretty apparent when looking at any document from the Edo era. It's all top to bottom, right to left. Remnants of it are also clear in temples where the signs above doorways are written right to left, not even top to bottom. Plus every Japanese novel and manga today is still written top to bottom right to left.

You are right, but it can be argued that during the time the painting was made, vertical writing was the predominant form, and I don't know whether horizontal writing was a thing at the time in Japan...

That said, as I implied in my other reply, the whole idea is a bit silly...