The OPNsense project is looking for help translating the OPNsense User Interface into the Japanese language. Simply ask for a login to the Pootle translation server, or, submit Pull Requests directly to OPNsense in GitHub.
Discussions regarding this work should happen in the OPNsense Japanese forum!
The simplest way to contribute, is to use the OPNsense translslation server, (Pootle).
OPNsense also welcomes translations directly from the project repo,
$ git clone https://github.com/opnsense/core.git $ cd core # vim lang/ja_JP.po
Translate the empty 'msgid' lines, and submit Pull Requests to the OPNSense team!
One warning, keep your commits and work short, and commit lines asap- it would be very sad if someone else was translating the very same lines you were- and time was wasted!