Add Japanese translation

[dalance]

This PR adds

• Japanese translation resource

  • This is based on rust-lang-ja/rust-by-example-ja@baf7bf0.
  • The original translators are below.

• Additional translation guide for Japanese

  • To avoid review about Japanese at this repo, rust-lang-ja members will review it at our organization's repo.
  • After the review, we open PR to this repo, so you can merge without Japanese review.
  • This guide describes the step for the review.

• Update mdbook-i18n-helpers to v0.3.1

  • This repo is affected by the issue: Fix codeblock in codeblock google/mdbook-i18n-helpers#129.

The original translators list:

@joemphilips @kdnakt @tatsuya6502 @dalance @a2not @h1g0 @swnakamura @etoal83 @TAKOYAKING8 @sei0o @todays-mitsui @furon-kuina @KANIYONIKA @baseballyama @eldesh @kotet @ohno418 @hnakai0909 @4geru @botamochi0x12 @sadnessOjisan @kakubin @takahiro-itazuri @isudzumi @naoty @kishiguro @KisaragiEffective @kazu728 @tomoyuki-nakabayashi @yoichi @H1Gdev @akiradeveloper @akiomik @uenojp @KeenS @stny @shiomiyan @r6eve @tos-kamiya

[rustbot]

r? @marioidival

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[marioidival]

Hello @dalance I have doubt about this line... That change could break the build, couldn't?

[dalance]

The build artifact seems to work fine.
What point is your consideration?

[marioidival]

My point is about the other languages. Unfortunately, I'm not fully aware of how the build process for other languages works and it seemed "strange" to add only the Japanese language to the variable, thus being able to influence it and there is also this comment that says that the change could only happen in the index.hbs file

[dalance]

LANGUAGES represents which translated language should be built.
So the current value is ja because there is Japanese resource only, and it will be like LANGUAGES: ja de fr po when other language resources are added.
en is not listed in the variable because it is the original language.

The comment means "When a new language is added to LANGUAGES, index.hbs should be updated too".
(It may be better that the comment is changed to the above sentence)
In index.hbs, the items of language selector are defined.
So If a new language is added, it should be added to the language selector.

[KisaragiEffective]

🚀