Add Japanese translations

[jayhesselberth]

After potools::po_create("ja")

Prompt:

You are an experienced translator. Create new a new translation for Japanese in po/R-ja.po. These are GNU gettext translations for pkgdown, a tool used to generate websites for R packages.

Translation advice:

- For languages that use genedered nouns, do your best to generate inclusive translations, e.g. "Beitragende:r" or "auteur·rice".
- If you need to change the order of substititions (e.g. %s) from English version, use %1$s to identify the first substituion %2$s, etc. Only use this form when the translated word order is different.
- You should choose to be succinct and contextually-correct even when there is not a precise translation.
- Use the "Last-Translator" field to document the tool (e.g. "Positron Assistant") and language model used for the translation (e.g. "Claude Sonnet 4.5")

[github-actions]

🚀 Deployed on https://68f6d34ae3474691e8d25267--pkgdown-dev.netlify.app

[shikokuchuo]

A few improvements.