syncLinks parses the default-locale content; processLinks resolves
each [[link]] against the current locale at render time. Link labels
preserve original spelling; destination resolves to the same document
in the current locale (with fallback).
Adds translations/defaultTranslation relations, current-locale accessors
with fallback to default_locale, isFallback/availableLocales helpers,
and search scope that delegates to DocumentTranslation.