Directory language/japanese

Some (by no means all) Japanese-related packages are named based on
the following prefix rule to indicate engine dependency:

  - BX... => works with any engine
  - PX... => (u)ptex-specific; a few of those work with luatexja too
  - ZX... => works with xetex only

We hope this helps demystify the naming.

Packages developed outside the Japanese community don't use this
convention.