Added Japanese translations

[MizuhoOkimoto]

This fixes #20
I added ja file under _locales folder and Japanese translations for all strings that are indexed by "message".
If I need to improve, please let me know. If it's okay, I will add the new version to the Changelog.
Thank you so much 😃

[Crease29]

Hi @MizuhoOkimoto , thank you very much for your contribution! 🤗
Could you please check your fork? I think there are missing some messages, that I've just added with commit e5d5d7c.
Would you be able to add them?

It would also be cool, if you add this on top of the changelog in the README.md in your pull request 😊

- **UPCOMING**
    - Added Japanese translations. Thanks to [@MizuhoOkimoto](https://github.com/MizuhoOkimoto)

Thanks a lot!

[Crease29]

@MizuhoOkimoto are you still working on this one? :)

[MizuhoOkimoto]

Hi @Crease29 , I'm so sorry for being late! I fetched your changes and updated README.md as you mentioned above. I appreciate it if you can review them. Thank you so much and happy holidays!

[Crease29]

All goo @MizuhoOkimoto , I'm also sorry for answering so late! Happy new year and thanks again for your contribution! That helps a lot :)

[MizuhoOkimoto]

Thank you for reviewing, @Crease29 !
No, worries! Holidays are busy😄 Happy new year!