I have property files for several locales, for example
locale/en_US, locale/en_GB, locale/es_ES, and locale/es. Running
from within FlexBuilder I can see the various locales just fine.
However when creating the .swf file using ANT and the <mxmlc>
task, only en_US is available. I've attached a snippet from my ANT
page, there is a <compiler.library-path> node that seems to
import a bundle directory that contains locales. I believe
<compiler.library-path> is from FlexAnt, so you would need to
enable it if you haven't.
Also look on the next page from the link I posted, there is
another example on how to import bundles in an automation.