Comment 8 for bug 428927

A workaround that lets you select your preferred languages by manipulating the list of recently selected language (note that the complete list in "other languages" is still empty)

sudo gedit /var/lib/gdm/.gconf/apps/gdm/simple-greeter/%gconf.xml

you will see something like this:

 <entry name="recent-languages" mtime="1255452885" type="list" ltype="string">
  <li type="string">
   <stringvalue>de_DE.UTF-8</stringvalue>
  </li>
 </entry>

just add another list item:

 <entry name="recent-languages" mtime="1255452885" type="list" ltype="string">
  <li type="string">
   <stringvalue>de_DE.UTF-8</stringvalue>
  </li>
  <li type="string">
   <stringvalue>ja_JP.UTF-8</stringvalue>
  </li>
 </entry>