checkconfig looks in the
underlaydirs, but plugins that
add underlays typically do so in their own
As far as I can see, this will result in it not adding translated versions
of underlays added by a plugin that comes after it in
for instance, if you have
add_plugins => qw(po smiley), you'll probably
not get the translated versions of
smileys.mdwn. (I haven't tested this.)
It doesn't happen because smiley adds the underlay unconditionally on import. Which is really more usual.
To see them all,
po should use
last => 1 when registering the hook.