Even though the ebuilds (3.1.1 and 3.2.0) contain a RDEPEND for kochi-substitute when using linguas_ja the LANG variable itself does not contain ja and thus the language pack is not installed. Simply adding ja to LANG and the problem was solved. Test on local system where the package was fetched from one of OOo's mirrors. app-office/openoffice ebuilds have ja in LANG, so I was just wondering why openoffice-bin doesn't. Reproducible: Always
The problem here is that upstream releases langpacks in a very inconsistent manner, some languages are there for one RC / release and gone with the next, so I have to manually check the list for each and every release... Which inevitably leads to missing some stuff, unfortunately, sorry about that. Anyway: Added ja (and ga which was also missing) to the supported linguas, thanks for reporting. Closing