Hugo currently exposes goldmark's built-in CJK extension in its goldmark markup config. But as discussed in yuin/goldmark#529, there are some shortcomings with using just this extension.

For example, the existing CJK extension doesn't handle the emphasis correctly:
Git **（ギット）**Hub

is transformed to:
<p>Git **（ギット）**Hub</p>

even though it should be transformed to:
<p>Git <strong>（ギット）</strong>Hub</p>

@tats-u has created a lot of extensions for existing Markdown libraries to support CJK emphasis https://github.com/tats-u/markdown-cjk-friendly/.
And he recently also created an extension for goldmark: https://github.com/tats-u/goldmark-cjk-friendly.

I believe it would be beneficial to include this extension directly in Hugo so that users could build CJK websites in Hugo 🙂