mnihongo is a fairly clumsy troff post-processor that 
catches unicode characters that might be japanese,
looks them up in a font file (currently
/lib/font/bit/pelm/unicode.9x24.font) to get the
bits, converts that to postscript, and dumps that
in the output.

Widths are taken from troff font Jp, which has at
best a simple approximation to the truth.

The program itself is normally called implicitly
by loading the troff macro package -mnihongo, which
does a .pi command.