Thanks for pointing this out. I've been working on a text extractor in Go at work and tried for a long time to get UnRTF working with RTF files containing Japanese characters to no avail. This lib lists catdoc as the extractor they use for RTF, so I'm going to give that a try.

Edit: Looks like catdoc doesn't work with RTF files containing Japanese characters either. Might end up having to use libreoffice or something like that.