OTF is a big spec, and if we support it or not depends in what is inside the otf file. We normally support otf fonts, but FlexCel doesn't support subsetting CFF fonts, so if there are CFF fonts inside the otf, subsetting won't work. (and while you could use the font without subsetting, that would result in huge files)
I've been looking at the noto* otf files and indeed, they use CFF inside.
So sadly those aren't supported. I just checked, and Excel doesn't support them either. If you export a file using those fonts to pdf from Excel (not using FlexCel at all), the text will be exported as images, not real text. You can see that the "used fonts" are empty, and if you edit the pdf, the text is just an image:
I can't really suggest alternatives myself because I don't speak any of those languages, but I am sure there are many korean, japanese, etc fonts which aren't CFF. Say, I know ms normally uses batang for korean: Batang font family - Typography | Microsoft Docs
but I don't know how good the font is, or the licensing issues with it. Someone who speaks korean will likely know better than I could. Same for the other languages.