We are currently working on finilizing the Japanese translations and will only be able to claim that one as tested in Designer 2.2. There are more translation files which were done by community that we haven't tested out (would be nice if someone could verify / help with that).
To enable a specific locale settings right now in Designer you have to modify its web.xml, look for:
<init-param>
<param-name>designer.locale</param-name>
<param-value>en_us</param-value>
</init-param>
and change to ja_jp for Japanese and deploy the war again. Note however that these translation files do not include stencil set translations. We are working on that for Japanese for the next release.