Make Speech in Safari read different languages
I would say this is a bug in macOS, so I've reported it.

But there's a simple workaround: use Automator to create a simple Service to speak the selected text in a voice of your choice.



You may also want to make sure to download the enhanced quality versions of the voice you choose to use through System Preferences > Accessibility:



If you select some text and option- or right-click to bring up the context menu, choose the Services menu item and you will see your custom Speak (Japanese) service that you can choose to read the text in that language with your chosen voice: