Update ja-jp translations and add japanese fonts (#746)

### Prerequisites

- [x] ~This pull request fixes a bug.~
- [x] This pull request adds a feature.
- [x] ~This pull request introduces breaking change.~

### Description

Add Japanese font and Update Japanese translations. Like #555 .

### Issues Resolved

- Strings that should be displayed in Japanese fonts are displayed in
Chinese fonts.
- If Japanese is written in Chinese fonts, it is difficult for Japanese
people to read.
- Used different languages for singular and plural. en-us and ja-jp.
- Wrong translation, `auther = "筆者"` is NG.
  - `auther = "著者"` is good.
  - `writer = "筆者, 執筆者"` is good.
- `powered_by` can usually be understood in English by Japanese people
who need to read that part.

### Checklist

#### General

- [x] Describe what changes are being made
- [x] Explain why and how the changes were necessary and implemented
respectively
- [x] Reference issue with `#<ISSUE_NO>` if applicable

#### Resources

- [x] If you have changed any SCSS code, run `make release` to
regenerate all CSS files

#### Contributors

- [x] Add yourself to `CONTRIBUTORS.md` if you aren't on it already