Add ja_jp translation

[virusbb001]

I created Japanese (ja_JP) translations, but I am not good at lean nor mathematics so I need supervising translators.

Once the remaining tasks are complete, I will switch to "Ready".

TODOs:

• Add ja_JP translation to lean4game (I will create pull request later)
• Ask someone to supervise the translation
• Should I squash all commits? > no need to squash commits,

[joneugster]

(btw, no need to squash commits, the entire PR will be squashed into one commit when merging)

[virusbb001]

To be honest, feedback from the Japanese LEAN community indicated that the translation quality was poor.
Fortunately, a community member recommended an introductory guide to LEAN that aligns with NNG4, so we are currently re-translating the material using that as a reference.
However, compounded by the fact that I have become busy, the re-translation is expected to take some time to complete.
Since leaving a pull request open and inactive for an extended period is not ideal, I am considering closing it for the time being.

[TentativeConvert]

I think it's better if you leave it open, at least as a draft, so that other potential translators can more easily find this thread and see that you're working on this.