Japanese and Chinese characters
 

I have a number of Chinese and Japanese translated novels which often have footnotes etc which have Chinese and Japanese character in them ie explanation of translation etc. However they don't display at all in the book at all, despite being able to see them when checking the epub in calibre.

I'm assuming the issue is that the kobo doesn't have a font to display them in. Is there a way for me to add one? Or the the issue possibly something else?

You'll have to change the font in the font menu or install your own. The built in Kobo Mincho or Kakugo fonts should display the characters, or you may have to switch to the built in Chinese font (AR something or other; don't have my reader on me) to view the Chinese-specific kanji. Otherwise if they still don't display the right characters or you're unhappy with how the font renders Western characters, create a 'fonts' folder on the device and copy over your preferred CJK font.

And if you are sideloading the books, you will probably need to sideload as kepubs. That should handle Japanese and Chinese characters better than the epub renderer.

Your Kobo should have several fonts for CJK characters. AR UDJingxihei, Kobo Tsukushi Mincho and Kobo UD Kagugo. I seem to remember the latter two are oriented towards Japanese while the first is oriented to Chinese. Selecting one of these fonts should allow you to see the text. There is also the old Code2000 font which is the most complete free font I've seen.

As an alternative, if you convert them to kepub, your Kobo should locate missing glyphs using other Kobo system fonts. Sometimes the results look a bit strange but still should be readable.

As yet another alternative, if you have an older Windows system with Office, it may have the Arial Unicode MS font installed which has a very comprehensive glyph selection. This may be on a bit shakey grounds legally but as long as it for your personal use, you should be safe enough.

There's also Google's Noto CJK fonts which use up the entire 65535 glyph limit in the font specifications. If it can't render any of the Asian characters, then the issue is something else.

Ahh, that's the issue - I was using Georgia for my font. Switching to Mincho gives me the characters and keeps the western text in serif which I prefer.

It's oddly light though? It doesn't look like the text as actually black, more like 80% grey?

You can use the Advanced link in the font menu to artificially add some weight (i.e. Make it bolder). Otherwise, you'll need to copy over a different font. Noto CJK Serif might be a good choice. There are different weight selections you can try.

Another option might be Adobe's Source Han CJK Serif fonts.

A potential issue is that, AFAIR, the kepub renderer only looks in Kobo's system fonts for alternative glyphs so a sideloaded font would not help unless I was using that font for reading. The other personal issue is that I'm not that fond of the Noto fonts for reading.

Hmm...I don't have an advanced option for Mincho like I do for Georgia. The dropdown fills the whole row.

Maybe I'll have to look at sideloading something because the lightness makes it a bit of a pain to read.

You could also apply the Enable advanced settings for all fonts patch from the patch thread for whichever firmware version you're running which would then give you access to the Advance link for Mincho, but if you don't want to go that route, then yeah, you'll need to sideload something. Good news is that there's lots of free and commercial font choices out there.