You are quite right.  It was my misunderstanding.

The right byte sequence of
もしもPlan 9で日本語を使いたいなら、私
are
[E3 82 82][E3 81 97][E3 82 82][Plan 9][E3 81 A7][E6 97 A5]
[E6 9C AC][E8 AA 9E][E3 82 92][E4 BD BF][E3 81 84][E3 81 9F]
[E3 81 84][E3 81 AA][W3 82 89][E3 80 81][E7 A7 81].

Anyway, thank you very much Russ, now we can use UTF-8 on your
Wiki server.  I replaced the bogus one.

Kenji