messages.po should contain all strings that have been set up 
for internationalization.  After translation, it will need to 
be compiled into a .mo file and put in an appropriate subdirectory
for the /i18n/locale/ directory

messages.js should contain all strings from javascript files that
have been set up for internationalization.  After translation, but
a copy of the file in the appropriate subdirectory of the /i18n/locale/ 
directory.  This files does not need to be compiled.

I have not yet automated the process for generating the messages.js
file using xgettext or other utilizies.  I have included a
getjsmessages.php script which extracts the javascript messages, but
it doesn't not currently merge in existing translated messages.  
Additional work needs to be done on this.