You can do CJK with degraded quality using only simple cmap processing. There's another layer that requires more sophistication: dealing with Han unification, processing variation selectors correctly, composing Hangul jamo.

If people want their software to work well across the world, they should not use simple libraries like this. Unfortunately, software that gets it right is complex and messy. See "Text Rendering Hates You" for a taste of some of the issues, or some of my blogging about skribo (I tried to solve some of this for Rust but ran out of time on my contract).