add Japanese translations

[Tsumagari]

#180 I added ja.json and /resources/lang/ja/validation.php

[kitbs]

どうもありがとう @Tsumagari
There are a few lines which have not been translated. Can you please fix them?

[Tsumagari]

I'm very happy for your fast, kindly response!
I've forgotten that I left a few lines, I'll fix them soon!
Before that, can I ask you a question?
I wonder "Hold up!" means like "stop! (or, freeze!)" or so, but, could it be "lift (something)" ?
Which mean should I choose to translate?

[kitbs]

Thank you again for your work!

The "Hold up!" translation is used on the 403 error page along with the funny "The government won't let us show you what's behind these doors" message, so yes, it means something like "Stop!" or "Wait!" to tell the user they are not allowed to access that page.

[Tsumagari]

I understand. I like these humor!
Thank you very much!

[garyross]

"Remember Me": "ログイン状態を保持", <-- this is better I think

Remember Me": "私を覚えておく", <-- this is a little odd for the functionality