Add Japanese locale

[SousiOmine]

I wanted this software to be available in Japanese, so I'm currently forking the repository and working on adding a Japanese translation.

The repository I'm working on is here (https://github.com/SousiOmine/sourcegit/tree/add_locale_ja_JP), but due to my lack of translation skills, lack of Git skills, and limited time I can devote to the work, it will be some time before I'm able to submit a pull request.

Finally, I'd like to thank the team who are creating this wonderful Git client.

(I apologize if this text sounds unnatural because I used translation software for it.)