papers: end-to-end i18n on papers.ac index — fetch translations.json, add Japanese
Switcher only swapped 19 of 39 papers because index.html carried a stale
hand-maintained translations object alongside the auto-generated
translations.json. Page now fetches the JSON at runtime, setLang reads
{title, subtitle} and re-appends the format/link suffix from the English
detail (e.g. "· arXiv 5pp", "· <a>interactive timeline</a>"), and papers
without translations fall back to English instead of half-swapping.
Also extended cleanLatex (\acrandname{}, Spanish accents, Danish letters
in {\x}/\x{}/\x\<sp> forms + LaTeX space-gobble), added Japanese to the
dropdown + /ja /jp redirects, and deferred the initial path-language
swap until the JSON has loaded so first paint isn't English-flash.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>