[proposal] Add unit test for Japanese language

[ryotsun]

I propose to add unit test for Japanese language.
It may help countries which have multi-byte characters like east asian countries.

At the beginning, I added unit test for Japanese language referred from German's.
After this is approved, I will also add more Japanese-specific unit test.

[miyauchi]

+1

It is very nice for us.

I want to open some tickets to improve some enhancement for Chinese, Korean and Japanese after this ticket will be merged.
For example, there is half-width space and full-width space in those languages, so we should be able to use full-width space as a separator of search keyword.

[miyauchi]

Related:
​https://meta.trac.wordpress.org/ticket/3163

[ryotsun]

Related ticket: https://core.trac.wordpress.org/ticket/44296

[miyauchi]

We wrote a unit test for WP Multibyte Patch on GitHub.
​https://github.com/jawordpressorg/automated-testing-for-wordpress-ja

There are descriptions what is multibyte patch doing in README.md on this repository.
(There are no official documentation in English for this plugin.)

I want to open some tickets to add tests based on the repo after this ticket will be merged.

Related: ​https://meta.trac.wordpress.org/ticket/3163

[miyauchi]

There is a list of the locales that include custom plugin in the package.
​https://meta.trac.wordpress.org/ticket/3163#comment:10

[johnbillion]

In 43359:

I18N: Introduce unit tests for the Japanese language in order to facilitate future improvements.

Props ryotsun

Fixes #43829