fix: improve unnatural Japanese and translate missing strings

[kakik0u]

I fixed some unnatural phrasing and translated the missing parts.
In addition, I removed several expressions that were clearly unnecessary and had been added only to the Japanese version, despite not being in the original text.

[marcprux]

Thanks! Please verify that it looks OK to you: https://keepandroidopen.org/ja/

[kakik0u]

Sorry, I ran a typo check with Claude and caught three that I missed, so I just submitted another PR(#393) to cover them.

[7654jp]

Hello, and sorry for interruption.
I wanted to say thank you for correcting my mistake.
Now it is more natural than mine. I think it's wonderful.
I'm just a silly student, and I didn't get the Star Wars reference (I've never seen it, actually).
So, let me excuse about the several expressions that were clearly unnecessary and had been added only to the Japanese version.
I thought that 'Paying a fee to Google' is required only when the developers wanted to release their apps for more than 20 people, according to Google's official document, also mentioned in #324 .
It is my bad that I didn't say anything about this when I made a PR.
But I hope that we can have a discussion about this.
Thank you.