Update po4a.conf and regenerate .po files.

Also, I've corrected src/doc/README.md in line with this change.

Signed-off-by: OGINO Masanori <masanori.ogino@gmail.com>
auto merge of #12005 : omasanori/rust/update-trans, r=brson

Also, I've corrected src/doc/README.md in line with this change.