Don't forget Right-To-Left languages, that also affects how UI elements are arranged (position within the page) and rendered (input widgets like sliders get reversed).
I think the currently blessed CSS solution is to only use {inline,block}-{start,end} https://developer.mozilla.org/en-US/docs/Web/CSS/CSS_logical... in place of {left,right,top,bottom} which then automagically supports even vertical scripts like Traditional Mongolian, but most people probably don't think that far ahead when just starting out.
I know Traditional Mongolian comes up regularly in i18n contexts, but I've never taken the time to see if it's something an app should support, based on how actively it is used. It seems like it's too minor, mostly due to the fact that nobody could be bothered to support it in electronic formats.
Vertical Japanese is very common in the print magazines I read, and is RTL amongst the rest of the Japanese text which is LTR. I just don't see it much online because of the trouble of setting it.
If I ever have too much spare time I must try to support vertical Mongolian in my app.