The OpenStreetMap Carto installation instructions have documentation on what Mapnik needs to properly render some scripts and they recommend
For proper rendering of non-latin scripts, particularly those with complicated diacritics and tone marks the requirements are
- FreeType 2.6.2 or later for CJK characters
- A recent enough version of Noto with coverage for the scripts needed.
Looking on maps1001, we have FreeType 2 2.5.2-3+deb8u2 which is known to have bugs with CJK.
I wonder why we're not having really obvious problems hitting this bug. Perhaps we're using an ancient version of the Noto CJK fonts?