fonts with ending .otf
 

I downloaded some Chinese fonts. They have the ending .otf

I have found the easiest way to get fonts on my computer is download the font, copy it into /usr/share/fonts/somefolder then

sudo fc-cache -rv

However, the fonts with the ending .otf are not picked up by this.

Any tips please?

.otf fonts are opentype fonts. I think it is neccesary to include the font installed directory in configuration file located at /etc/fonts/fonts.conf or /etc/fonts/conf.d/somefiles.

This is odd: I have otf fonts which work perfectly.

Do they work if installed in $HOME/.fonts ? There shouldn't be a difference, but computers can be funny things.

You could also try
sudo mkfontdir /usr/share/fonts/somefolder
sudo mkfontscale /usr/share/fonts/somefolder
I never use these nowadays but, again, you never know!

These commands are not needed for truetype and opentype fonts.

They were needed for older fonts distributed with XFree86/Xorg.

/etc/fonts/conf.d/fonts.conf is an xml file, which opens in Firefox when I click it.

I can't see anything to make it include .otf

All the files in /etc/fonts/conf.d/ and /etc/fonts/conf.avail/ are xml files.

Any more tips out there please?

Open them in text editor. They are usually well commented so that you can add in easily.

Also can you just copy/symlink the otf file to a directory which gets it font included already.