Busybox is only used for scripting and command-line tools. If you want to support Chinese in the user interface, it is gmenu2x
that you should be customizing.
For now the font path is hardcoded at the top of src/font.cpp (#define TTF_FONT), but it would be better to make that a configurable option, preferably with different fonts for different languages.
gmenu2x already uses unicode, but things like word wrapping might need to be changed for displaying Chinese properly.
You can start a custom-built menu at boot by creating a script /usr/local/sbin/frontend_start, for example:
This starts a gmenu2x executable from the home dir instead of from the rootfs; adjust the path if necessary.