This will apply to *ALL* plugins in *ALL* languages, so should probably only be used when doing site customization...
You'd probably do:
$config['site']['locale_path'] = '/srv/awesome/data/locale';
$config['plugins']['locale_path'] = '/srv/awesome/data/locale';
with a structure like:
srv/
awesome/
data/
locale/
en/
LC_MESSAGES/
statusnet.po
OpenID.po
AnonymousFave.po
etc, all alongside each other. You could separate plugins from the core if you like.
Where locale files have not already been generated, you can build one for a plugin like so:
php scripts/update_po_templates.php --plugin=MyPlugin
and pull out the template file:
plugins/MyPlugin/locale/MyPlugin.pot
Edit that (make sure you at least set the CHARSET, probably to UTF-8) and save your customized .po
files into the structure as above, and use msgfmt to generate .mo files for final output.