po-debconf is only for managing packages, not for packaged software and there is no ja.po in https://github.com/ledgersmb/LedgerSMB/tree/master/locale/po
so it is normal to not showing non-existent choice in the selection as most likely there is no one who will translate the 10000 lines of program po into ja, if you dont like this mismatch, you can just disable the po for debconf for years (not *for a while*) I don't think it will be done before Debian bts bug counts 1M -- victory no need to CC me :-)
