docs: add Japanese (i18n) builds for HTML and latexpdf

[ttadano]

Summary

Adds an English/Japanese multi-language option to the Sphinx docs for both HTML and latexpdf, built on Sphinx's native gettext i18n. The build language is selected via the DOCS_LANGUAGE env var (default en); untranslated strings fall back to English, so the Japanese site is usable immediately and fills in as catalogs are translated.

What's included

• conf.py — language from DOCS_LANGUAGE; locale_dirs + gettext_compact. For Japanese: latex_engine='lualatex' with the LuaTeX-ja classes (ltjsbook/ltjsarticle) and polyglossia/babel disabled (their \setmainlanguage{japanese} otherwise aborts against the Latin body font). Registers the language-switcher assets.
• Makefile — html-ja, latexpdf-ja, update-po targets.
• requirements.txt — sphinx-intl for catalog management.
• languages.txt — per-branch source of truth (first entry = default) that CI reads.
• _static/ — new language-switcher.{js,css}; version-switcher.js updated for the nested <version>/<lang>/ layout.
• locale/ja/ — 24 scaffolded gettext catalogs (empty msgstr, ready to translate); one sample entry filled in (Users Guide → ユーザーガイド) to exercise the pipeline.
• docs.yml — per-version language matrix → <version>/<lang>/ + per-version languages.json + landing redirect; PR validation builds every language; new Build Japanese PDF job (LuaLaTeX) uploads the PDF as an artifact. Branches without languages.txt keep the existing single-language layout (fully backward compatible).

Translator workflow

cd docs
make update-po     # refresh source/locale/ja catalogs after docs change
# …fill in msgstr "" in source/locale/ja/LC_MESSAGES/*.po…
make html-ja       # preview Japanese HTML
make latexpdf-ja   # Japanese PDF via lualatex

Verified locally (venv = pinned requirements + sphinx-intl)

• English & Japanese HTML build clean; translated string renders only in ja, no leak into en.
• Japanese PDF: 157 pages via LuaLaTeX, CJK glyphs render, 0 missing-glyph warnings.
• Published layout/manifests correct; both switchers render and reflect state in a real browser (日本語 on /ja/, English on /en/).
• docs.yml parses; the bash language-matrix logic yields ["en","ja"] and the right dirs/redirects.

Reviewer notes

1. The CI Japanese-PDF job needs a first-run sanity check. The LaTeX config and build are verified locally, but I can't run Actions here to confirm Ubuntu's fonts-freefont-otf is picked up by luaotfload for the Latin body font. If not, the fix is a one-line font override in conf.py (point \setmainfont at Harano Aji).
2. The PDF is uploaded as a CI artifact, not published to the Pages site — kept off the deploy path to avoid touching the working multi-version deploy. Easy to wire into the site (e.g. …/<version>/ja/ALAMODE.pdf) as a follow-up if wanted.

🤖 Generated with Claude Code