Hello,
I discovered the same bug that prevents you from changing Japanese text to italic in the iOS version, appears in the Desktop version.
Here is the link to my original post about the iOS version.
https://forum.literatureandlatte.com/t/japanese-text-italic-only-cannot-be-selected/34493/1
Note that Bold and Underline works without any problem.
Looking forward to your further guidance.
Cheers! 
Darren
Does the Japanese font you are using have an italic variant. I use Chinese and many Chinese fonts don’t, so clicking italic has no effect. Word and some other word-processors generate a faux-italic by slanting the regular glyph; Scrivener—or any Mac editor based on the Apple text engine—doesn’t. You can check by trying it in TextEdit.
Hope that helps.
Mark
Hi Mark,
I checked by pasting the text in TextEdit. You are right. There does not seem to be a italic variation for the font.
Actually, I was not aware of this issue at all. You have opened my eyes.
I guess my options are either to possibly find a font with a italic variation, export what I write to Word and edit it or hope that Keith can offer some sort of faux-italic. I think I will try finding a font with a italic variation.
The search continues …
Darren
Hi Darren,
Glad to have been of help so far. I’ve checked on my Mac and none of the Japanese fonts I have—or Chinese for that matter—include an italic variant, which doesn’t surprise me. In your position, I wouldn’t bother with italic, but would change font to a more handwritten or italic-like font. So, I generally use something like STSong as my main font, so I’d then use STKaiti as my “italic” variant, or if I wanted something that would stand out even more, I’d use the more cursive Xingkai SC. If you go through your Japanese fonts in FontBook, you should be able to identify suitable combinations.
You can then use a preset to allocate each of your fonts and set up appropriate short-cuts. You can also set up paragraph presets which don’t impact the font selection again with appropriate shortcuts. Not quite so easy as Cmd-i, but still no great difficulty.
I don’t know whether there are CJK fonts available which include italic variants.
Cheers.
Mark
EDIT: You would have to import the fonts onto your iOS device if they are not on it already, and use Format > Formatting > Export Presets for iOS and then import those (instructions in the “Other files” section of the “Sync’ing” document of the tutorial in Scrivener for iOS, if you haven’t done all that already). Alternatively, when you’ve copied the fonts into Dropbox, use FontManager (free app) on your iOS device, to install them in the System, as they will then be available in any other apps like Pages, Notebooks, etc. on the device.