Add Japanese translation

[shotadft]

It's simple, but I've added a Japanese translation.

[opekope2]

Hello!
Thank you for your patience — I was working on updating the translations, which took more time than I expected. I updated the translations on the dev branch with over 70 new translations.
If you want to submit updated translations for the next OptiGUI 3 alpha release, you have at least 30 days.

[opekope2]

I added some new translations today because I forgot them earlier.

[opekope2]

After implementing more features, I had to make some minor changes to the translations — currently, there are 87 of them.

[opekope2]

The translation keys for OptiGUI 3 alpha 4 are finished, no more additions, maybe small clarifications.
Many translation keys were added since your last revision (88 in total).
You have at least 30 days to update your translation.

For optigui.rp_loader.warn.no_interaction_target, here's some context. blocks, entities, items, inventory, and unknown will be part of V2 JSON resources (just like texture.PATH in OptiFine properties), so it makes sense to keep those 5 words as-is:

{
  "blocks": [],
  "entities": [],
  "items": [],
  "inventory": false,
  "unknown": true
}

[shotadft]

I've been busy with other work for a while, so I'll fix it soon.

[shotadft]

It's been a while. I've updated the Japanese translation.