Comment 1 for bug 476678

Related earlier reports:
Bug #167486 “Spanish on osx” (Fix released)
Bug #167849 “Localisation of UI unavailable on MacOS X” (Fix released)
Bug #349825 “Download Current Mac Version is French Only!” (Incomplete)
Bug #360317 “Brazilian localization don't launch!” (Incomplete)

A different language selection mechanism is described in:
Bug #242105 “Versión en Español para Mac” (Fix committed)
If this solution works for all localizations, one could update the packages to include an empty directory 'Contents/Resources/ja.lprj' or similar.